IconMan: suppress -Wformat-overflow=

The compiler is unable to determine if these values are never null.
Although it's unlikely to be encountered, provide fallback values in the
call to ConsoleDebug that uses them directly.

Signed-off-by: Matt Jolly <kangie@gentoo.org>

modules/FvwmIconMan/winlist.c

@@ -113,8 +113,11 @@ static int matches_string (NameType type, char *pattern, char *tname,
 
   ConsoleDebug (WINLIST, "matches_string: type: 0x%x pattern: %s\n",
 		type, pattern);
-  ConsoleDebug (WINLIST, "\tstrings: %s:%s %s:%s\n", tname, iname,
-		rname, cname);
+  ConsoleDebug (WINLIST, "\tstrings: %s:%s %s:%s\n",
+                tname ? tname : "(null)",
+                iname ? iname : "(null)",
+                rname ? rname : "(null)",
+                cname ? cname : "(null)");
 
   if (tname && (type == ALL_NAME || type == TITLE_NAME)) {
     ans |= matchWildcards (pattern, tname);