pcbnew crashes when selecting Measure tool

Description

When freezing a move operation to check distances, pcbnew can be made to crash by repeatedly switching between move and measure. Note that I don't have confirmation yet that the current behavior of ALT is as-designed, but it's useful to me (see #6317 (closed)).

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Steps to reproduce

1. M move something (footprint or track)
2. ALT
3. select Measure tool
4. (apparently optional: click, move, click to create ruler)
5. M move
6. ALT
7. select Measure tool

KiCad Version

Version: (5.99.0-6862-g9454f9df), release build, Nov 6 2020 22:01:21

Application: Pcbnew

Version: (5.99.0-6862-g9454f9df92), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.71.0 OpenSSL/1.1.1g (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.41.0

Platform: Windows 10 (build 19041), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Nov  6 2020 22:01:21
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.73.0
	OCE: 6.9.1
	Curl: 7.71.0
	ngspice: 32
	Compiler: GCC 10.2.0 with C++ ABI 1014

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
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON