Fixed allocation of the buffer for the file extension.

source/texk/web2c/mplibdir/mp.w

@@ -34928,7 +34928,7 @@ extreme cases so it may have to be shortened on some systems.
 @<Use |c| to compute the file extension |s|@>=
 {
-  s = xmalloc (13, 1);
+  s = xmalloc (14, 1);
   mp_snprintf (s, 13, ".%i", (int) c);
   s[13]='\0';
 }