Silence "missing contrecord" error.

Commit dd38ff28ad added a new error message "missing contrecord" when
we fail to reassemble a record.  Unfortunately that caused noisy
messages to be logged by pg_waldump at end of segment, and by walsender
when asked to shut down on a segment boundary.

Remove the new error message, so that this condition signals end-of-
WAL without a message.  It's arguably a reportable condition that should
not be silenced while performing crash recovery, but fixing that without
introducing noise in the other cases will require more research.

Back-patch to 15.

Reported-by: Tomas Vondra <tomas.vondra@enterprisedb.com>
Discussion: https://postgr.es/m/6a1df56e-4656-b3ce-4b7a-a9cb41df8189%40enterprisedb.com

src/backend/access/transam/xlogreader.c

@@ -914,15 +914,11 @@ err:
 		state->missingContrecPtr = targetPagePtr;
 
 		/*
-		 * If we got here without reporting an error, report one now so that
-		 * XLogPrefetcherReadRecord() doesn't bring us back a second time and
-		 * clobber the above state.  Otherwise, the existing error takes
-		 * precedence.
+		 * If we got here without reporting an error, make sure an error is
+		 * queued so that XLogPrefetcherReadRecord() doesn't bring us back a
+		 * second time and clobber the above state.
 		 */
-		if (!state->errormsg_buf[0])
-			report_invalid_record(state,
-								  "missing contrecord at %X/%X",
-								  LSN_FORMAT_ARGS(RecPtr));
+		state->errormsg_deferred = true;
 	}
 
 	if (decoded && decoded->oversized)