Crash in MacOS HighSierra in footprint editor when cut or delete silk segment (lp:#1796309)

[Original report](https://bugs.launchpad.net/bugs/1796309) created by **Kyle Crane (photomankc)**
```
When using the footprint editor in OS X High Sierra, if I place a line
segment or pad in the silk layer and later either delete or cut that
segment or pad then KiCad crashes.  I can reproduce this behavior in any
symbol library.  This may be similar to the bug 1795195, but my stack trace
did not seem to involve the function referenced in that fix.  If this is
the same issue I apologize.

This issue is not seen when editing a PCB however.

Application: kicad
Version: (6.0.0-rc1-dev-634-gc8704e82e), debug build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.54.0 LibreSSL/2.0.20 zlib/1.2.11 nghttp2/1.24.0
Platform: Mac OS X (Darwin 17.7.0 x86_64), 64 bit, Little endian, wxMac
Build Info:
    wxWidgets: 3.0.4 (UTF-8,STL containers,compatible with 2.8)
    Boost: 1.67.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.51.0
    Compiler: Clang 8.0.0 with C++ ABI 1002

Build settings:
    USE_WX_GRAPHICS_CONTEXT=ON
    USE_WX_OVERLAY=ON
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON

Process:               kicad [44030]
Path:
/private/var/folders/*/kicad.app/Contents/MacOS/kicad
Identifier:            org.kicad-pcb.kicad
Version:               ??? (???)
Code Type:             X86-64 (Native)
Parent Process:        ??? [1]
Responsible:           kicad [44030]
User ID:               1235584508

Date/Time:             2018-10-04 22:55:12.126 -0500
OS Version:            Mac OS X 10.13.6 (17G65)
Report Version:        12
Bridge OS Version:     3.0 (14Y664)
Anonymous UUID:        82942D6F-6594-B835-7ACE-75B58B7EF130

Sleep/Wake UUID:       8D3FE03D-5620-4272-8958-BD2BCB46EE56

Time Awake Since Boot: 470000 seconds
Time Since Wake:       1000 seconds

System Integrity Protection: enabled

Notes:                 Translocated Process

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       EXC_I386_GPFLT
Exception Note:        EXC_CORPSE_NOTIFY

Termination Signal:    Segmentation fault: 11
Termination Reason:    Namespace SIGNAL, Code 0xb
Terminating Process:   exc handler [0]

Application Specific Information:
Performing@ selector(clickedAction:) from sender wxNSMenuItem
0x6040006ddff0

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   _pcbnew.kiface                      0x000000011abe0200
KIGFX::VIEW::Hide(KIGFX::VIEW_ITEM*, bool) + 64
1   _pcbnew.kiface                      0x000000011a1f9f56
SELECTION_TOOL::unhighlight(BOARD_ITEM*, int, SELECTION&) + 166
2   _pcbnew.kiface                      0x000000011a1ea7e0
SELECTION_TOOL::unselect(BOARD_ITEM*) + 64
3   _pcbnew.kiface                      0x000000011a1f1631
SELECTION_TOOL::UnselectItems(TOOL_EVENT const&) + 449
4   _pcbnew.kiface                      0x000000011a2024ca int
std::__1::__invoke_void_return_wrapper<int>::__call<std::__1::__bind<int
(SELECTION_TOOL::*&)(TOOL_EVENT const&), SELECTION_TOOL*,
std::__1::placeholders::__ph<1>&>&, TOOL_EVENT const&>(std::__1::__bind<int
(SELECTION_TOOL::*&)(TOOL_EVENT const&), SELECTION_TOOL*,
std::__1::placeholders::__ph<1>&>&&&, TOOL_EVENT const&&&) + 538
5   _pcbnew.kiface                      0x000000011a2021f9
std::__1::__function::__func<std::__1::__bind<int
(SELECTION_TOOL::*&)(TOOL_EVENT const&), SELECTION_TOOL*,
std::__1::placeholders::__ph<1>&>, std::__1::allocator<std::__1::__bind<int
(SELECTION_TOOL::*&)(TOOL_EVENT const&), SELECTION_TOOL*,
std::__1::placeholders::__ph<1>&> >, int (TOOL_EVENT
const&)>::operator()(TOOL_EVENT const&) + 57
6   _pcbnew.kiface                      0x000000011ac55831
std::__1::function<int (TOOL_EVENT const&)>::operator()(TOOL_EVENT const&)
const + 145
7   _pcbnew.kiface                      0x000000011ac5577d COROUTINE<int,
TOOL_EVENT const&>::callerStub(long) + 109
8   _pcbnew.kiface                      0x000000011abd392a make_fcontext +
42
9   ???                                 0x000000013bfc07b0 0 + 5301340080

Thread 1:: com.apple.NSEventThread
0   libsystem_kernel.dylib              0x00007fff74b0020a mach_msg_trap +
10
1   libsystem_kernel.dylib              0x00007fff74aff724 mach_msg + 60
2   com.apple.CoreFoundation            0x00007fff4cb65785
__CFRunLoopServiceMachPort + 341
3   com.apple.CoreFoundation            0x00007fff4cb64ad7 __CFRunLoopRun +
1783
4   com.apple.CoreFoundation            0x00007fff4cb64153
CFRunLoopRunSpecific + 483
5   com.apple.AppKit                    0x00007fff4a23cfc4 _NSEventThread +
184
6   libsystem_pthread.dylib             0x00007fff74cd1661 _pthread_body +
340
7   libsystem_pthread.dylib             0x00007fff74cd150d _pthread_start +
377
8   libsystem_pthread.dylib             0x00007fff74cd0bf9 thread_start +
13

Thread 2:: com.apple.CFSocket.private
0   libsystem_kernel.dylib              0x00007fff74b09cf2 __select + 10
1   com.apple.CoreFoundation            0x00007fff4cb9f88b
__CFSocketManager + 635
2   libsystem_pthread.dylib             0x00007fff74cd1661 _pthread_body +
340
3   libsystem_pthread.dylib             0x00007fff74cd150d _pthread_start +
377
4   libsystem_pthread.dylib             0x00007fff74cd0bf9 thread_start +
13

Thread 3:
0   libsystem_kernel.dylib              0x00007fff74b0a28a
__workq_kernreturn + 10
1   libsystem_pthread.dylib             0x00007fff74cd1009
_pthread_wqthread + 1035
2   libsystem_pthread.dylib             0x00007fff74cd0be9 start_wqthread +
13

Thread 4:
0   libsystem_pthread.dylib             0x00007fff74cd0bdc start_wqthread +
0
1   ???                                 0x537465731b117f00 0 +
6013542947496230656

Thread 5:
0   libsystem_pthread.dylib             0x00007fff74cd0bdc start_wqthread +
0
1   ???                                 0x0000000100000038 0 + 4294967352

Thread 0 crashed with X86 Thread State (64-bit):
  rax: 0x2000000000000000  rbx: 0x000000013bfc0678  rcx: 0x00007fcaa75019a0
rdx: 0x0000000000000000
  rdi: 0x00007fcaa75019a0  rsi: 0x00006040001c7170  rbp: 0x000000013b19cff0
rsp: 0x000000013b19cfc0
   r8: 0x000000011ac557fe   r9: 0x0000000000000009  r10: 0x000000000000000c
r11: 0x0000000000000000
  r12: 0x000000013b19d460  r13: 0x000000011ac55795  r14: 0x0000000000000000
r15: 0x0000600000cccd30
  rip: 0x000000011abe0200  rfl: 0x0000000000010206  cr2: 0x0000000109760000

Logical CPU:     0
Error Code:      0x00000000
Trap Number:     13


Binary Images:
       0x101fee000 -        0x10216dff7 +org.kicad-pcb.kicad (??? - ???)
<0F869D62-58A2-3918-B8A3-55137BEBF016>
/var/folders/*/kicad.app/Contents/MacOS/kicad
```
Original tags: crash delete editor footprint

Story points: 6.0