tests: fix minor memory leak in test_isis_lspdb

AddressSanitizer detected the isis structure allocated in main() was
never freed. This is a minor test code issue with no impact on isisd
itself.

Signed-off-by: Vincent Jardin <vjardin@free.fr>

Author: vjardin

tests/isisd/test_isis_lspdb.c

@@ -84,5 +84,6 @@ int main(int argc, char **argv)
 	struct isis *isis = NULL;
 	isis = calloc(1, sizeof(*isis));
 	test_lsp_build_list_nonzero_ht();
+	free(isis);
 	return 0;
 }