AddressSanitizer: heap-buffer-overflow in pilotfile::update_stats_backout()

This occurred when I clicked on "Accept" after playing SM3-06 from the techroom.

> Steps to Reproduce.
> 1. Utilise a version of Clang that supports AddressSantizer (https://code.google.com/p/address-sanitizer/wiki/AddressSanitizer).
> 2. Build with -fsanitize=address C/C++ flag
> 3. Run the game with mediavps 3.6.12, and Blue Plant WiH2. Play through Icarus until such time as a number of ships with rotating subsystems are on screen i.e. Karunas.

`==12331== ERROR: AddressSanitizer: heap-buffer-overflow on address 0x602c000605dc at pc 0xc18240 bp 0x7fff3b676ec0 sp 0x7fff3b676eb8
READ of size 4 at 0x602c000605dc thread T0
#0 0xc1823f in pilotfile::update_stats_backout(scoring_struct*, bool) /home/mememe/src/fs2open.github.com.niffiwan/code/pilotfile/pilotfile.cpp:290
0000001 0x90a8dd in debrief_close() /home/mememe/src/fs2open.github.com.niffiwan/code/missionui/missiondebrief.cpp:2094
0000002 0x41ef30 in game_leave_state(int, int) /home/mememe/src/fs2open.github.com.niffiwan/code/freespace2/freespace.cpp:5618
0000003 0x5b6c1c in gameseq_set_state(int, int) /home/mememe/src/fs2open.github.com.niffiwan/code/gamesequence/gamesequence.cpp:279
0000004 0x41e0c1 in game_process_event(int, int) /home/mememe/src/fs2open.github.com.niffiwan/code/freespace2/freespace.cpp:5184
0000005 0x5b7847 in gameseq_process_events() /home/mememe/src/fs2open.github.com.niffiwan/code/gamesequence/gamesequence.cpp:399
0000006 0x4218f5 in game_main(char*) /home/mememe/src/fs2open.github.com.niffiwan/code/freespace2/freespace.cpp:7153
0000007 0x421e45 in main /home/mememe/src/fs2open.github.com.niffiwan/code/freespace2/freespace.cpp:7288
0000008 0x7fc8f1b37ec4 in __libc_start_main /build/buildd/eglibc-2.19/csu/libc-start.c:287
0000009 0x40c6c8 in _start ??:?
0x602c000605dc is located 36 bytes to the right of 376-byte region [0x602c00060440,0x602c000605b8)
allocated by thread T0 here:
#0 0x7fc8f47044e5 in calloc ??:?
0000001 0x7fc8f36ee03b in glXCreateNewContext ??:?
Shadow bytes around the buggy address:
0x0c0600004060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c0600004070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 fa
0x0c0600004080: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00
0x0c0600004090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c06000040a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c06000040b0: 00 00 00 00 00 00 00 fa fa fa fa[fa]fa fa fa fa
0x0c06000040c0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c06000040d0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c06000040e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c06000040f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c0600004100: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Heap righ redzone: fb
Freed Heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack partial redzone: f4
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
ASan internal: fe
==12331== ABORTING`