Fix minor leak in pg_dump

Move allocation to after we check the remote server version, to avoid
a possible, very minor, memory leak.  This makes us more consistent
throughout as most places in pg_dump are done in the same way (due, in
part, to previous fixes like this).

Spotted by the Coverity scanner.

src/bin/pg_dump/pg_dump.c

@@ -3602,7 +3602,7 @@ getConversions(Archive *fout, int *numConversions)
 	PGresult   *res;
 	int			ntups;
 	int			i;
-	PQExpBuffer query = createPQExpBuffer();
+	PQExpBuffer query;
 	ConvInfo   *convinfo;
 	int			i_tableoid;
 	int			i_oid;
@@ -3617,6 +3617,8 @@ getConversions(Archive *fout, int *numConversions)
 		return NULL;
 	}
 
+	query = createPQExpBuffer();
+
 	/*
 	 * find all conversions, including builtin conversions; we filter out
 	 * system-defined conversions at dump-out time.