pcbnew: "Route->Tune lenght of a differential pair" crashes kicad. (lp:#1851251)

[Original report](https://bugs.launchpad.net/bugs/1851251) created by **Dino Ghilardi (dino-ghilardi)**
```
Trying to tune a diff pair length crashes the program.

To reproduce: Open the attached project and try to tune the length of the
diff pair.
--------------------------------

5.1.2: Not affected.
5.1.4: Not affected.
5.1.5rc1: Not affected

rev: 51ed01d765b681f30a7d2cfdfaba0d71c43a903c: affected


Note:
The build is marked "dirty" because the patch from Bug #1842906 has been
applied.

GDB messages:
Thread 1 "kicad" received signal SIGSEGV, Segmentation fault.
0x00007fffe0ed1a4c in PNS::NODE::FindJoint(VECTOR2<int> const&, int, int)
() from /home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface

Backtrace:
(gdb) backtrace
#0  0x00007fffe0ed1a4c in PNS::NODE::FindJoint(VECTOR2<int> const&, int,
int) () from /home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface
#1  0x00007fffe0ed48dc in PNS::NODE::FindLineEnds(PNS::LINE const&,
PNS::JOINT&, PNS::JOINT&) ()
   from /home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface
#2  0x00007fffe0ecd4f4 in
PNS::MEANDER_PLACER_BASE::GetTotalPadToDieLength(PNS::LINE const&) const ()
   from /home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface
#3  0x00007fffe0ea475e in PNS::DP_MEANDER_PLACER::Start(VECTOR2<int>
const&, PNS::ITEM*) ()
   from /home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface
#4  0x00007fffe0e6775b in PNS::ROUTER::StartRouting(VECTOR2<int> const&,
PNS::ITEM*, int) ()
   from /home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface
#5  0x00007fffe0e7fbe4 in LENGTH_TUNER_TOOL::performTuning() () from
/home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface
#6  0x00007fffe0e81714 in LENGTH_TUNER_TOOL::MainLoop(TOOL_EVENT const&) ()
from /home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface
#7  0x00007fffe10de6c0 in COROUTINE<int, TOOL_EVENT
const&>::callerStub(long) () from
/home/dinoghi/SANDBOXKICAD5/usr/bin/_pcbnew.kiface
#8  0x00005555556ec791 in make_fcontext ()
#9  0x00ffffff00ffffff in ?? ()
#10 0x00ffffff00ffffff in ?? ()
#11 0x00ffffff00ffffff in ?? ()
#12 0x00000000001b8f81 in ?? ()
#13 0x00007ffff3b11328 in main_arena () from /lib/x86_64-linux-
gnu/libc.so.6
#14 0x000055555a3fd690 in ?? ()
#15 0x000055555a3fd690 in ?? ()
#16 0x000055555a3fd690 in ?? ()
#17 0x00ffffff00ffffff in ?? ()
#18 0x00ffffff00ffffff in ?? ()
#19 0x00ffffff00ffffff in ?? ()
#20 0x00ffffff00ffffff in ?? ()
#21 0x00ffffff00ffffff in ?? ()
#22 0x00ffffff00ffffff in ?? ()
#23 0x00ffffff00ffffff in ?? ()
#24 0x00ffffff00ffffff in ?? ()
#25 0x00ffffff00ffffff in ?? ()
#26 0x00ffffff00ffffff in ?? ()
#27 0x00ffffff00ffffff in ?? ()
#28 0x00ffffff00ffffff in ?? ()
#29 0x00ffffff00ffffff in ?? ()
#30 0x00ffffff00ffffff in ?? ()
#31 0x00ffffff00ffffff in ?? ()
#32 0x00ffffff00ffffff in ?? ()
#33 0x00ffffff00ffffff in ?? ()
#34 0x00ffffff00ffffff in ?? ()
#35 0x00ffffff00ffffff in ?? ()
#36 0x00ffffff00ffffff in ?? ()
#37 0x00ffffff00ffffff in ?? ()
#38 0x00ffffff00ffffff in ?? ()
#39 0x00ffffff00ffffff in ?? ()

--------------------------------
Application: Pcbnew
Version: (5.99.0-290-g56525296b-dirty), release 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 4.9.0-8-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    Build date: Nov  4 2019 14:09:17
    wxWidgets: 3.0.2 (wchar_t,wx containers,compatible with 2.8) GTK+ 2.24
    Boost: 1.69.0
    OpenCASCADE Community Edition: 6.8.0
    Curl: 7.52.1
    Compiler: GCC 6.3.0 with C++ ABI 1010

Build settings:
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON
--------------------------------

Cheers,
Dino.
```
Original tags: pcbnew

Story points: 6.0