32x - fix crash, fixes #1826

Trigger condition:  Any 32x rom smaller than 4MB in total size that tries to read out of bounds between its actual end and the 4MB limit

I've fixed the emulator crash, but I do not know if such roms are actually mirroring correctly - they probably aren't

waterbox/picodrive/pico/cart.c

@@ -184,6 +184,11 @@ static unsigned char *PicoCartAlloc(int filesize, int is_sms)
     // align to 512K for memhandlers
     rom_alloc_size = (filesize + 0x7ffff) & ~0x7ffff;
   }
+  if (rom_alloc_size < 0x400000) {
+	// sh2 memory mapping assumes that there's at least this much readable memory
+	// The comment in that code is `0x3fffff; // FIXME`, but I guess it was never fixed
+    rom_alloc_size = 0x400000;
+  }
 
   if (rom_alloc_size - filesize < 4)
     rom_alloc_size += 4; // padding for out-of-bound exec protection