Fix determination of broken LSN in OVERWRITTEN_CONTRECORD

In commit ff9f111bce I mixed up inconsistent definitions of the LSN of
the first record in a page, when the previous record ends exactly at the
page boundary.  The correct LSN is adjusted to skip the WAL page header;
I failed to use that when setting XLogReaderState->overwrittenRecPtr,
so at WAL replay time VerifyOverwriteContrecord would refuse to let
replay continue past that record.

Backpatch to 10.  9.6 also contains this bug, but it's no longer being
maintained.

Discussion: https://postgr.es/m/45597.1637694259@sss.pgh.pa.us

src/backend/access/transam/xlogreader.c

@@ -464,7 +464,7 @@ restart:
 			 */
 			if (pageHeader->xlp_info & XLP_FIRST_IS_OVERWRITE_CONTRECORD)
 			{
-				state->overwrittenRecPtr = state->currRecPtr;
+				state->overwrittenRecPtr = RecPtr;
 				ResetDecoder(state);
 				RecPtr = targetPagePtr;
 				goto restart;