[Core/MD] fixed ROM size detection when enabling/disabling SRAM on power-on (fixes SRAM support in Hardball 95, NBA Hang Time and other 3MB games with SRAM mapped at 0x300000)

core/cart_hw/md_cart.c

@@ -380,7 +380,7 @@ void md_cart_init(void)
   if (sram.on && !sram.custom)
   {
     /* SRAM is mapped by default unless it overlaps with ROM area (Phantasy Star 4, Beyond Oasis/Legend of Thor, World Series Baseball 9x, Duke Nukem 3D,...) */
-    if (sram.start >= size)
+    if (sram.start >= cart.romsize)
     {
       m68k.memory_map[sram.start >> 16].base    = sram.sram;
       m68k.memory_map[sram.start >> 16].read8   = sram_read_byte;