Check version before allocating PQExpBuffer

In pg_dump.c:getEventTriggers, check what major version we are on
before calling createPQExpBuffer() to avoid leaking that bit of
memory.

Leak discovered by the Coverity scanner.

Back-patch to 9.3 where support for dumping event triggers was
added.

src/bin/pg_dump/pg_dump.c

@@ -5746,7 +5746,7 @@ EventTriggerInfo *
 getEventTriggers(Archive *fout, int *numEventTriggers)
 {
 	int			i;
-	PQExpBuffer query = createPQExpBuffer();
+	PQExpBuffer query;
 	PGresult   *res;
 	EventTriggerInfo *evtinfo;
 	int			i_tableoid,
@@ -5766,6 +5766,8 @@ getEventTriggers(Archive *fout, int *numEventTriggers)
 		return NULL;
 	}
 
+	query = createPQExpBuffer();
+
 	/* Make sure we are in proper schema */
 	selectSourceSchema(fout, "pg_catalog");