Fix possible failure to mark hash metapage dirty.

Report and suggested fix by Lixian Zou.  Amit Kapila put it
in the form of a patch and reviewed.

Discussion: http://postgr.es/m/151739848647.1239.12528851873396651946@wrigleys.postgresql.org

src/backend/access/hash/hashovfl.c

@@ -341,9 +341,10 @@ found:
 			metap->hashm_mapp[metap->hashm_nmaps] = BufferGetBlockNumber(newmapbuf);
 			metap->hashm_nmaps++;
 			metap->hashm_spares[splitnum]++;
-			MarkBufferDirty(metabuf);
 		}
 
+		MarkBufferDirty(metabuf);
+
 		/*
 		 * for new overflow page, we don't need to explicitly set the bit in
 		 * bitmap page, as by default that will be set to "in use".