pcbnew segfault when forming invalid polygon (lp:#1759024)

[Original report](https://bugs.launchpad.net/bugs/1759024) created by **Seth Hillbrand (sethh)**
```
While working on a separate issue, I encountered a segfault when moving a
polygon point to overlap an existing point.  It does not happen frequently.
Maybe about 1:100 when selecting points and moving them to overlap and then
re-moving them so that they no longer overlap.

If I click on the two overlapping points without dragging, it appears that
they are merged.  But just clicking and dragging the points over each other
does not merge them, so I can move them independently.

To reproduce, create a 5-segment copper zone and move the 5th point over
the 4th, fill, unfill, move the point again, wash, rinse, repeat.  Wish I
had a better procedure for this one.

Backtrace:
#0  0x00007fffcee3fe1c in POINT_EDITOR::OnSelectionChange
(this=0x55555911fa50, aEvent=...)
    at /home/seth/code/kicad/kicad-
launchpad/pcbnew/tools/point_editor.cpp:337
#1  0x00007fffcee4a488 in std::__invoke_impl<int, int (POINT_EDITOR::*
const&)(TOOL_EVENT const&), POINT_EDITOR*&, TOOL_EVENT const&> (__f=
   @ 0x555557de0010: (int (POINT_EDITOR::*)(POINT_EDITOR * const, const
TOOL_EVENT &)) 0x7fffcee3f93e <POINT_EDITOR::OnSelectionChange(TOOL_EVENT
const&)>, __t=@0x555557de0020: 0x55555911fa50, __args#0=...)
    at /usr/include/c++/6/functional:227
#2  0x00007fffcee49fdd in std::__invoke<int (POINT_EDITOR::*
const&)(TOOL_EVENT const&), POINT_EDITOR*&, TOOL_EVENT const&> (__fn=
   @ 0x555557de0010: (int (POINT_EDITOR::*)(POINT_EDITOR * const, const
TOOL_EVENT &)) 0x7fffcee3f93e <POINT_EDITOR::OnSelectionChange(TOOL_EVENT
const&)>, __args#0=@0x555557de0020: 0x55555911fa50, __args#1=...)
    at /usr/include/c++/6/functional:251
#3  0x00007fffcee49aaf in std::_Mem_fn_base<int
(POINT_EDITOR::*)(TOOL_EVENT const&), true>::operator()<POINT_EDITOR*&,
TOOL_EVENT const&> (this=0x555557de0010, __args#0=@0x555557de0020:
0x55555911fa50, __args#1=...)
    at /usr/include/c++/6/functional:604
#4  0x00007fffcee4935c in std::_Bind<std::_Mem_fn<int
(POINT_EDITOR::*)(TOOL_EVENT const&)> (POINT_EDITOR*,
std::_Placeholder<1>)>::__call<int, TOOL_EVENT const&, 0ul,
1ul>(std::tuple<TOOL_EVENT const&>&&, std::_Index_tuple<0ul, 1ul>) (
    this=0x555557de0010, __args=<unknown type in
/usr/local/bin/_pcbnew.kiface, CU 0x360b96e, DIE 0x366d4ba>)
    at /usr/include/c++/6/functional:934
#5  0x00007fffcee48c50 in std::_Bind<std::_Mem_fn<int
(POINT_EDITOR::*)(TOOL_EVENT const&)> (POINT_EDITOR*,
std::_Placeholder<1>)>::operator()<TOOL_EVENT const&, int>(TOOL_EVENT
const&) (this=0x555557de0010, __args#0=...)
    at /usr/include/c++/6/functional:993
#6  0x00007fffcee480e5 in std::_Function_handler<int (TOOL_EVENT const&),
std::_Bind<std::_Mem_fn<int (POINT_EDITOR::*)(TOOL_EVENT const&)>
(POINT_EDITOR*, std::_Placeholder<1>)> >::_M_invoke(std::_Any_data const&,
TOOL_EVENT const&) (
    __functor=..., __args#0=...) at /usr/include/c++/6/functional:1717
#7  0x00007fffcf4ad4d3 in std::function<int (TOOL_EVENT
const&)>::operator()(TOOL_EVENT const&) const (
    this=0x5555590e5908, __args#0=...) at
/usr/include/c++/6/functional:2127
#8  0x00007fffcf4aa33d in COROUTINE<int, TOOL_EVENT const&>::callerStub
(aData=140736610786112)
    at /home/seth/code/kicad/kicad-launchpad/include/tool/coroutine.h:329
#9  0x00007fffcf4fcda1 in make_fcontext () from
/usr/local/bin/_pcbnew.kiface
#10 0x00005555586373a0 in ?? ()
#11 0x0000555557aec5a0 in ?? ()
#12 0x0000000000000000 in ?? ()



Application: kicad
Version: (5.0.0-rc2-dev-312-g347f6dadd), debug build
Libraries:
    wxWidgets 3.0.2
    libcurl/7.52.1 GnuTLS/3.5.8 zlib/1.2.8 libidn2/0.16 libpsl/0.17.0
(+libidn2/0.16) libssh2/1.7.0 nghttp2/1.18.1 librtmp/2.3
Platform: Linux 4.9.0-6-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.62.0
    Curl: 7.52.1
    Compiler: GCC 6.3.0 with C++ ABI 1010

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_ACTION_MENU=OFF
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_SPICE=OFF
```
Original tags: pcbnew

Story points: 6.0