Fix 82kB leak of PlayField16 and PlayField8.

This commit fixes an 82 kilobyte leak in the Supaplex engine in Rocks 'n' Diamonds. The PlayField16 and PlayField8 variables were initialized in InitGlobals() but never freed.

Subtracting preceding_buffer_size and SP_MAX_PLAYFIELD_WIDTH from the buffers being freed is necessary because in InitGlobals (src/game_sp/Globals.c lines 169-179) the pointers to the buffers are moved forward by the corresponding amounts.

Here are the relevant bits from a Valgrind leak-check:

==10502==
==10502== 16,608 bytes in 1 blocks are possibly lost in loss record 2,388 of 2,467
==10502==    at 0x4C300A5: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==10502==    by 0x4CEA3B: checked_calloc (src/libgame/misc.c:1254)
==10502==    by 0x4FCE45: InitGlobals (src/game_sp/Globals.c:176)
==10502==    by 0x4FE1E2: Form_Load (src/game_sp/MainForm.c:58)
==10502==    by 0x4F7799: sp_open_all (src/game_sp/init.c:62)
==10502==    by 0x40F9C5: OpenAll (src/init.c:6133)
==10502==    by 0x40479E: main (src/main.c:7755)

...
==10502== 
==10502== 67,296 bytes in 1 blocks are possibly lost in loss record 2,414 of 2,467
==10502==    at 0x4C300A5: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==10502==    by 0x4CEA3B: checked_calloc (src/libgame/misc.c:1254)
==10502==    by 0x4FCE16: InitGlobals (src/game_sp/Globals.c:170)
==10502==    by 0x4FE1E2: Form_Load (src/game_sp/MainForm.c:58)
==10502==    by 0x4F7799: sp_open_all (src/game_sp/init.c:62)
==10502==    by 0x40F9C5: OpenAll (src/init.c:6133)
==10502==    by 0x40479E: main (src/main.c:7755)

ArtsoftEntertainment / rocksndiamonds

Fix 82kB leak of PlayField16 and PlayField8. #3