Check for NULL string before calling strlen(). Fixes the core dump

reported in #84873.

src/delete.c

@@ -266,7 +266,7 @@ io_from_ping_dialog (GIOChannel   *channel,
 
       meta_topic (META_DEBUG_PING,
                   "Read %d bytes strlen %d \"%s\" from child\n",
-                  len, strlen (str), str);
+                  len, str ? strlen (str) : 0, str ? str : "NULL");
       
       if (len > 0)
         {