Fix minor memory leak in pg_basebackup and pg_receivewal

The result of the query used to retrieve the WAL segment size from the
backend was not getting freed in two code paths.  Both pg_basebackup and
pg_receivewal exit immediately if a failure happened on this query, so
this was not an actual problem, but it could be an issue if this code
gets used for other tools in different ways, be they future tools in
this code tree or external, existing, ones.

Oversight in commit fc49e24f, so backpatch down to 11.

Author: Jie Zhang
Discussion: https://postgr.es/m/970ad9508461469b9450b64027842331@G08CNEXMBPEKD06.g08.fujitsu.local
Backpatch-through: 11

src/bin/pg_basebackup/streamutil.c

@@ -313,9 +313,12 @@ RetrieveWalSegSize(PGconn *conn)
 	if (sscanf(PQgetvalue(res, 0, 0), "%d%s", &xlog_val, xlog_unit) != 2)
 	{
 		pg_log_error("WAL segment size could not be parsed");
+		PQclear(res);
 		return false;
 	}
 
+	PQclear(res);
+
 	/* set the multiplier based on unit to convert xlog_val to bytes */
 	if (strcmp(xlog_unit, "MB") == 0)
 		multiplier = 1024 * 1024;
@@ -334,7 +337,6 @@ RetrieveWalSegSize(PGconn *conn)
 		return false;
 	}
 
-	PQclear(res);
 	return true;
 }