Fix edge-case for xl_tot_len broken by bae868ca.

bae868ca removed a check that was still needed.  If you had an
xl_tot_len at the end of a page that was too small for a record header,
but not big enough to span onto the next page, we'd immediately perform
the CRC check using a bogus large length.  Because of arbitrary coding
differences between the CRC implementations on different platforms,
nothing very bad happened on common modern systems.  On systems using
the _sb8.c fallback we could segfault.

Restore that check, add a new assertion and supply a test for that case.
Back-patch to 12, like bae868ca.

Tested-by: Tom Lane <tgl@sss.pgh.pa.us>
Tested-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGLCkTT7zYjzOxuLGahBdQ%3DMcF%3Dz5ZvrjSOnW4EDhVjT-g%40mail.gmail.com

src/backend/access/transam/xlogreader.c

@@ -380,6 +380,15 @@ restart:
 	}
 	else
 	{
+		/* There may be no next page if it's too small. */
+		if (total_len < SizeOfXLogRecord)
+		{
+			report_invalid_record(state,
+								  "invalid record length at %X/%X: wanted %u, got %u",
+								  (uint32) (RecPtr >> 32), (uint32) RecPtr,
+								  (uint32) SizeOfXLogRecord, total_len);
+			goto err;
+		}
 		/* We'll validate the header once we have the next page. */
 		gotheader = false;
 	}
@@ -795,6 +804,8 @@ ValidXLogRecord(XLogReaderState *state, XLogRecord *record, XLogRecPtr recptr)
 {
 	pg_crc32c	crc;
 
+	Assert(record->xl_tot_len >= SizeOfXLogRecord);
+
 	/* Calculate the CRC */
 	INIT_CRC32C(crc);
 	COMP_CRC32C(crc, ((char *) record) + SizeOfXLogRecord, record->xl_tot_len - SizeOfXLogRecord);

src/test/recovery/t/039_end_of_wal.pl

@@ -287,6 +287,19 @@ ok( $node->log_contains(
 		$log_size),
 	"xl_tot_len short");
 
+# xl_tot_len in final position, not big enough to span into a new page but
+# also not eligible for regular record header validation
+emit_message($node, 0);
+$end_lsn = advance_to_record_splitting_zone($node);
+$node->stop('immediate');
+write_wal($node, $TLI, $end_lsn, build_record_header(1));
+$log_size = -s $node->logfile;
+$node->start;
+ok( $node->log_contains(
+		"invalid record length at .*: wanted 24, got 1", $log_size
+	),
+	"xl_tot_len short at end-of-page");
+
 # Need more pages, but xl_prev check fails first.
 emit_message($node, 0);
 $end_lsn = advance_out_of_record_splitting_zone($node);