metalink: fix improbable crash parsing metalink filename

The this_url pointer wasn't being initialized, so if strdup() would return
null when copying the filename in a metalink file, then hilarity would
ensue during the cleanup phase. This change was brought to you by clang,
which noticed this and raised a warning.

src/tool_operate.c

@@ -535,7 +535,7 @@ int operate(struct Configurable *config, int argc, argv_item_t argv[])
         long retry_numretries;
         long retry_sleep_default;
         long retry_sleep;
-        char *this_url;
+        char *this_url = NULL;
         int metalink_next_res = 0;
 
         outfile = NULL;