Plug another leak.

If cf_export_specified_packets() succeeds, and it wrote to a temporary
file, it leaks the name of the file to which it was writing.  Free that
after we've renamed that file on top of the target file (safe save).

file.c

@@ -4902,6 +4902,7 @@ cf_export_specified_packets(capture_file *cf, const char *fname,
       cf_rename_failure_alert_box(fname, errno);
       goto fail;
     }
+    g_free(fname_new);
   }
 
   return CF_WRITE_OK;