fix memory leak

valgrind warning:
==3751== 11,956 (88 direct, 11,868 indirect) bytes in 1 blocks are definitely lost in loss record 196 of 196
==3751==    at 0x4C277AB: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==3751==    by 0x4E61BF4: SDL_CreateRGBSurface (in /usr/lib64/libSDL-1.2.so.0.11.4)
==3751==    by 0x54F8FAC: IMG_LoadPNG_RW (in /usr/lib64/libSDL_image-1.2.so.0.8.4)
==3751==    by 0x54F4EC7: IMG_LoadTyped_RW (in /usr/lib64/libSDL_image-1.2.so.0.8.4)
==3751==    by 0x888652: InitializeEngine() (in ./valyriatear)
==3751==    by 0x6031CE: main (in ./valyriatear)
Merge pull request #407 from lieff/master

fix memory leak