pg_amcheck: PQclear query results

While the potential memory leak is small, ensure to PQclear the query
results before disconnecting.

Author: Jiao Shuntian <312199339@qq.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/tencent_F34922C91C41E76C734773E767C9FBDB9906@qq.com

src/bin/pg_amcheck/pg_amcheck.c

@@ -587,6 +587,7 @@ main(int argc, char *argv[])
 			/* Querying the catalog succeeded, but amcheck is missing. */
 			pg_log_warning("skipping database \"%s\": amcheck is not installed",
 						   PQdb(conn));
+			PQclear(result);
 			disconnectDatabase(conn);
 			conn = NULL;
 			continue;