Fix buffer overflow when creating firmware.dfc file string

When the user uses an external firmware image, there is a 
call to getpathnoext() in firmware.cpp:571 in order to
generate a file name for the saved firmware config file.

The size of the MMU.fw.userfile char buffer is only 260,
as declared in mc.h:73. However, getpathnoext() expects
the buffer size to be MAX_PATH, which is declared as 1024
in types.h:77. The buffer overflow occurs in path.h:293
with the call to strncpy(), which copies 1024 chars into
MMU.fw.userfile, which is only 260 chars.

This patch fixes this bug by setting the MMU.fw.userfile
char buffer to a size of MAX_PATH. This is consistent with
the char buffers used for the other file names.

From rogerman, #3328686

desmume/src/mc.h

@@ -1,6 +1,6 @@
 /*  Copyright (C) 2006 thoduv
     Copyright (C) 2006 Theo Berkau
-	Copyright (C) 2008-2010 DeSmuME team
+    Copyright (C) 2008-2011 DeSmuME team
 
     This file is part of DeSmuME
 
@@ -70,7 +70,7 @@ struct memory_chip_t
 	
 	// needs only for firmware
 	bool isFirmware;
-	char userfile[260];
+	char userfile[MAX_PATH];
 };
 
 //the new backup system by zeromus