pg_dump: fix double free of query results

This bug was introduced while refactoring in commit 1631598e --- no need
to back-patch.

Bug report and fix from Joachim Wieland.

src/bin/pg_dump/pg_dump.c

@@ -2372,8 +2372,6 @@ dumpBlobs(Archive *fout, void *arg)
 		PQclear(res);
 	} while (ntups > 0);
 
-	PQclear(res);
-
 	return 1;
 }