Reset unused fields in GIN data leaf page footer.

The maxoff field is not used in the new, compressed page format. Let's
reset it when converting an old-format page to the new format. The code
won't care either way, but this makes it possible to use the field for
something else in the future.

src/backend/access/gin/gindatapage.c

@@ -817,7 +817,10 @@ dataPlaceToPageLeafRecompress(Buffer buf, disassembledLeaf *leaf,
 	}
 	Assert(newsize <= GinDataLeafMaxContentSize);
 	GinDataLeafPageSetPostingListSize(page, newsize);
-	GinPageSetCompressed(page);	 /* in case it was in pre-9.4 format before */
+
+	/* Reset these in case the page was in pre-9.4 format before */
+	GinPageSetCompressed(page);
+	GinPageGetOpaque(page)->maxoff = InvalidOffsetNumber;
 
 	/* Put WAL data */
 	recompress_xlog.length = (uint16) newsize;

src/backend/access/gin/ginxlog.c

@@ -153,6 +153,7 @@ ginRedoRecompress(Page page, ginxlogRecompressDataLeaf *data)
 	memcpy(segment, data->newdata, data->length - data->unmodifiedsize);
 	GinDataLeafPageSetPostingListSize(page, data->length);
 	GinPageSetCompressed(page);
+	GinPageGetOpaque(page)->maxoff = InvalidOffsetNumber;
 }
 
 static void