NewFork at nonexistent time

[pleroy]

Found at commit 60e70b7 while exercising #463, at very high warp (1000x? 10000x?) when stopping/starting the plugin repeatedly. This didn't show up at lower warp (100x).

Log line format: [IWEF]mmdd hh:mm:ss.uuuuuu threadid file:line] msg
    @   2831A327    google::LogMessageFatal::~LogMessageFatal [0x2831A326+102] (c:\users\phl\projects\github\google\glog-0.3.3\src\logging.cc:1995)
    @   282E9847    principia::physics::Trajectory<principia::geometry::Frame<enum principia::serialization::Frame_PluginTag,3,1> >::NewFork [0x282E9846+230] (c:\users\phl\projects\github\principia\physics\trajectory_body.hpp:165)
    @   282FDEFE    principia::ksp_plugin::Plugin::UpdatePredictions [0x282FDEFD+349] (c:\users\phl\projects\github\principia\ksp_plugin\plugin.cpp:773)
    @   282F7300    principia::ksp_plugin::Plugin::AdvanceTime [0x282F72FF+1007] (c:\users\phl\projects\github\principia\ksp_plugin\plugin.cpp:177)
    @   282D7A41    principia__AdvanceTime [0x282D7A40+80] (c:\users\phl\projects\github\principia\ksp_plugin\interface.cpp:237)
F0323 22:46:42.413811  4460 trajectory_body.hpp:165] Check failed: fork_it != timeline_.end() NewFork at nonexistent time

INFO file

[pleroy]

Reproduced in a unit test, this happens when we fork at the end of a trajectory (for the prolongation) and fork immediately at the end of that fork (for the prediction) without adding any point in between.