Initialize pad bytes in GinFormTuple().

Every other core buffer page consumer initializes the bytes it furnishes
to PageAddItem().  For consistency, do the same here.  No back-patch;
regardless, we couldn't count on the fix so long as binary upgrade can
carry forward affected index builds.

src/backend/access/gin/ginentrypage.c

@@ -112,6 +112,12 @@ GinFormTuple(GinState *ginstate,
 	if (newsize != IndexTupleSize(itup))
 	{
 		itup = repalloc(itup, newsize);
+		/*
+		 * PostgreSQL 9.3 and earlier did not clear this new space, so we
+		 * might find uninitialized padding when reading tuples from disk.
+		 */
+		memset((char *) itup + IndexTupleSize(itup),
+			   0, newsize - IndexTupleSize(itup));
 
 		/* set new size in tuple header */
 		itup->t_info &= ~INDEX_SIZE_MASK;