#2235 SIGSEV simgear::canvas::Group::updateImpl

[legoboyvdlp]

A slightly different one today, from the same user.
threaded-gc enabled, I asked to test with disabled.

[James Turner]

Stack trace:

VM Regions Near 0xa3b51613af8:
MALLOC_LARGE_REUSABLE 00000002a83d2000-00000002a93d2000 [ 16.0M] rw-/rwx SM=PRV
-->
MALLOC_NANO 0000600000000000-0000600008000000 [128.0M] rw-/rwx SM=PRV

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0 org.flightgear.mac-nightly 0x0000000104418b09 simgear::canvas::Group::updateImpl(double) + 137
1 org.flightgear.mac-nightly 0x0000000104418b30 simgear::canvas::Group::updateImpl(double) + 176
2 org.flightgear.mac-nightly 0x0000000104418b30 simgear::canvas::Group::updateImpl(double) + 176
3 org.flightgear.mac-nightly 0x000000010445480b simgear::canvas::Canvas::update(double) + 1659
4 org.flightgear.mac-nightly 0x0000000104351d06 simgear::PropertyBasedMgr::update(double) + 86
5 org.flightgear.mac-nightly 0x000000010439c40e SGSubsystemGroup::Member::update(double) + 110
6 org.flightgear.mac-nightly 0x000000010439ba9d SGSubsystemGroup::update(double) + 781
7 org.flightgear.mac-nightly 0x00000001043a0286 SGSubsystemMgr::update(double) + 118
8 org.flightgear.mac-nightly 0x0000000104127503 0x103afe000 + 6460675
9 org.flightgear.mac-nightly 0x00000001040a7d3a fgOSMainLoop() + 186
10 org.flightgear.mac-nightly 0x0000000104126d2d fgMainInit(int, char**) + 4989
11 org.flightgear.mac-nightly 0x0000000103b08309 main + 329