5.99 placing arc tracks on arc tracks often leads to crash

Description

When you edit arc tracks by placing new track on existing ones (in the hope that router removes the extras) sometimes it will lead to a crash.

Steps to reproduce

place_arc_for_editing_crash

place_arc_for_editing_crash_2

KiCad Version

Application: KiCad PCB Editor

Version: (5.99.0-11293-gc11ee69499), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.77.0 OpenSSL/1.1.1k zlib/1.2.11 zstd/1.5.0 libidn2/2.3.1 libpsl/0.21.1 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.43.0

Platform: Linux 5.10.42-1-MANJARO x86_64, 64 bit, Little endian, wxGTK, KDE, x11

Build Info:
	Date: Jul  4 2021 12:39:56
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.76.0
	OCC: 7.5.0
	Curl: 7.77.0
	ngspice: 34
	Compiler: GCC 11.1.0 with C++ ABI 1016

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON