Fix use of free in walsender error handling after a sysid mismatch.

Found via valgrind. The bug exists since the introduction of the walsender,
so backpatch to 9.0.

Andres Freund

src/backend/replication/libpqwalreceiver/libpqwalreceiver.c

@@ -135,6 +135,7 @@ libpqrcv_connect(char *conninfo, XLogRecPtr startpoint)
 			 GetSystemIdentifier());
 	if (strcmp(primary_sysid, standby_sysid) != 0)
 	{
+		primary_sysid = pstrdup(primary_sysid);
 		PQclear(res);
 		ereport(ERROR,
 				(errmsg("database system identifier differs between the primary and standby"),