Make memory read return 0x90 instead of 0xFF in the impossible(?) case where something read from an impossible location

oswan/source/memory.cpp

@@ -227,7 +227,7 @@ BYTE cpu_readmem20(DWORD addr)
       return ws_rom[temp];      
    }
 
-   return(0xff);
+   return(0x90);
 }
 ////////////////////////////////////////////////////////////////////////////////
 //