Add a missing null pointer check in wc_unescape, to bring it in line

with the usage comment saying you're allowed to pass NULL to find out
only the return value. No caller actually does pass NULL at the
moment.

[originally from svn r9920]

wildcard.c

@@ -326,7 +326,8 @@ int wc_unescape(char *output, const char *wildcard)
 	    wildcard++;
 	}
     }
-    *output = '\0';
+    if (output)
+        *output = '\0';
     return 1;			       /* it's clean */
 }