pg_dump: add missing "destroyPQExpBuffer(query)" in dumpForeignServer().

Coverity complained about this resource leak (why now, I don't know,
since it's been like that a long time).  Our general policy in pg_dump
is that PQExpBuffers are worth cleaning up, so do it here too.  But
don't bother with a back-patch, because it seems unlikely that very
many databases contain enough FOREIGN SERVER objects to notice.

src/bin/pg_dump/pg_dump.c

@@ -14287,6 +14287,7 @@ dumpForeignServer(Archive *fout, ForeignServerInfo *srvinfo)
 	destroyPQExpBuffer(q);
 	destroyPQExpBuffer(delq);
 	destroyPQExpBuffer(labelq);
+	destroyPQExpBuffer(query);
 }
 
 /*