[CRASHFIX] Stop vehicle missing frame check from causing crash by using typical part iteration loop. Optimize too.

[atomicdryad]

• Fix crash in vehicle::add_missing_frames

* Prevent vehicle::add_missing_frames from needlessly triggering precalc_mounts/find_* etc multiple times

gcc linux with -O3 results in:

Program received signal SIGSEGV, Segmentation fault.
vehicle::add_missing_frames (this=0x8972df0) at vehicle.cpp:99
99 int next_y = it->mount_dy;
(gdb) bt
#0 vehicle::add_missing_frames (this=0x8972df0) at vehicle.cpp:99
#1 0x0853d0f2 in vehicle::load (this=0x8972df0, stin=...) at vehicle.cpp:84
#2 0x082913a7 in mapbuffer::unserialize (this=0x865a040, fin=...) at mapbuffer.cpp:412
#3 0x08291a2a in mapbuffer::load (this=0x865a040) at mapbuffer.cpp:248
#4 0x0827c27c in main (argc=0, argv=0xbffff7a4) at main.cpp:87

(gdb)

This fixes the crash in -O3 and unoptimized.

[ianestrachan]

Odd - I tested this by pulling an old version, saving the world, then loading it in the new version, and it added the extra frames...

...but now I see it looks like I was modifying a list while iterating over it. Ouch. Yeah, that would definitely cause problems. Thanks for fixing this.