Assert that pre/post-fix updated tuples are on the same page during replay.

If they were not 'oldtup.t_data' would be dereferenced while set to NULL
in case of a full page image for block 0.

Do so primarily to silence coverity; but also to make sure this prerequisite
isn't changed without adapting the replay routine as that would appear to
work in many cases.

Andres Freund

src/backend/access/heap/heapam.c

@@ -8115,11 +8115,13 @@ newsame:;
 
 	if (xlrec->flags & XLOG_HEAP_PREFIX_FROM_OLD)
 	{
+		Assert(samepage);
 		memcpy(&prefixlen, recdata, sizeof(uint16));
 		recdata += sizeof(uint16);
 	}
 	if (xlrec->flags & XLOG_HEAP_SUFFIX_FROM_OLD)
 	{
+		Assert(samepage);
 		memcpy(&suffixlen, recdata, sizeof(uint16));
 		recdata += sizeof(uint16);
 	}