pg_waldump: Fix small memory leak when rmgr->rm_identify returns NULL.

This got broken in 604f7956, shortly after rm_identify's
introduction.

Author: Andres Freund
Discussion: https://postgr.es/m/20191029233341.4gnyau7e5v2lh5sc@alap3.anarazel.de
Backpatch: 9.5, where rm_identify was introduced

src/bin/pg_waldump/pg_waldump.c

@@ -517,17 +517,18 @@ XLogDumpDisplayRecord(XLogDumpConfig *config, XLogReaderState *record)
 
 	XLogDumpRecordLen(record, &rec_len, &fpi_len);
 
-	id = desc->rm_identify(info);
-	if (id == NULL)
-		id = psprintf("UNKNOWN (%x)", info & ~XLR_INFO_MASK);
-
 	printf("rmgr: %-11s len (rec/tot): %6u/%6u, tx: %10u, lsn: %X/%08X, prev %X/%08X, ",
 		   desc->rm_name,
 		   rec_len, XLogRecGetTotalLen(record),
 		   XLogRecGetXid(record),
 		   (uint32) (record->ReadRecPtr >> 32), (uint32) record->ReadRecPtr,
 		   (uint32) (xl_prev >> 32), (uint32) xl_prev);
-	printf("desc: %s ", id);
+
+	id = desc->rm_identify(info);
+	if (id == NULL)
+		printf("desc: UNKNOWN (%x) ", info & ~XLR_INFO_MASK);
+	else
+		printf("desc: %s ", id);
 
 	/* the desc routine will printf the description directly to stdout */
 	desc->rm_desc(NULL, record);