free cache_tmpfile in tns_free (#485)

doesn't really matter since the process is about to exit
anyways, but makes it consistent with everything else.

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/485

thumbs.c

@@ -198,6 +198,8 @@ CLEANUP void tns_free(tns_t *tns)
 
 	free(cache_dir);
 	cache_dir = NULL;
+	free(cache_tmpfile);
+	cache_tmpfile = cache_tmpfile_base = NULL;
 }
 
 static Imlib_Image tns_scale_down(Imlib_Image im, int dim)