pcbnew: crash when tuning length of track (cadstar imported board)

Description

A crash occurs when trying to tune the length at the joint between two arc tracks, as shown in the video below:

arc_track_crash

Steps to reproduce

1. Open pcbnew
2. Import -> Non-KiCad board file. minimal_route_offset_curved_track.cpa
3. Attempt to tune the length of the track at the intersection between the two curved tracks.

KiCad Version

Application: KiCad PCB Editor

Version: (5.99.0-10149-gb2f788adc1), release build

Libraries:
	wxWidgets 3.1.4
	libcurl/7.74.0-DEV Schannel zlib/1.2.11

Platform: Windows 10 (build 18363), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Apr  6 2021 00:06:49
	wxWidgets: 3.1.4 (wchar_t,STL containers)
	Boost: 1.75.0
	OCC: 7.5.0
	Curl: 7.74.0-DEV
	ngspice: 34
	Compiler: Visual C++ 1928 without C++ ABI

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON