Fix end-of-loop optimization in pglz_find_match() function.

After the recent pglz optimization patch, the next/prev pointers in the
hash table are never NULL, INVALID_ENTRY_PTR is used to represent invalid
entries instead. The end-of-loop check in pglz_find_match() function didn't
get the memo. The result was the same from a correctness point of view, but
because the NULL-check would never fail, the tiny optimization turned into
a pessimization.

Reported by Stephen Frost, using Coverity scanner.

src/backend/utils/adt/pg_lzcompress.c

@@ -466,7 +466,7 @@ pglz_find_match(int16 *hstart, const char *input, const char *end,
 		 * Be happy with lesser good matches the more entries we visited. But
 		 * no point in doing calculation if we're at end of list.
 		 */
-		if (hent)
+		if (hent != INVALID_ENTRY_PTR)
 		{
 			if (len >= good_match)
 				break;