Add additional safety check against invalid backup label file

It was already checking for invalid data after "BACKUP FROM", but
would possibly crash if "BACKUP FROM" was missing altogether.

found by Coverity

src/backend/access/transam/xlog.c

@@ -9481,7 +9481,7 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive)
 	 * during the backup.
 	 */
 	ptr = strstr(remaining, "BACKUP FROM:");
-	if (sscanf(ptr, "BACKUP FROM: %19s\n", backupfrom) != 1)
+	if (!ptr || sscanf(ptr, "BACKUP FROM: %19s\n", backupfrom) != 1)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));