Fix sram saving and loading for some games

Some games (eg. Sonic 3 & Knuckles) have sram but the core doesn't set
"sram.start" and "sram.end". In any case, this constant is used in
source/win/main.c.

Signed-off-by: Saggi Mizrahi <ficoos@gmail.com>

libretro/libretro.c

@@ -892,7 +892,7 @@ size_t retro_get_memory_size(unsigned id)
    switch (id)
    {
       case RETRO_MEMORY_SAVE_RAM:
-         return sram.end - sram.start;
+         return 0x10000;
 
       default:
          return 0;