pg_waldump: Fix error message for WAL files smaller than XLOG_BLCKSZ.

When opening a WAL file smaller than XLOG_BLCKSZ (e.g. 0 bytes long) while
determining the wal_segment_size, pg_waldump checked errno, despite errno not
being set by the short read. Resulting in a bogus error message.

Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/20220214.181847.775024684568733277.horikyota.ntt@gmail.com
Backpatch: 11-, the bug was introducedin fc49e24f

src/bin/pg_waldump/pg_waldump.c

@@ -204,15 +204,12 @@ search_directory(const char *directory, const char *fname)
 									 WalSegSz),
 							fname, WalSegSz);
 		}
-		else
-		{
-			if (errno != 0)
+		else if (r < 0)
 			fatal_error("could not read file \"%s\": %m",
 						fname);
 		else
 			fatal_error("could not read file \"%s\": read %d of %zu",
 						fname, r, (Size) XLOG_BLCKSZ);
-		}
 		close(fd);
 		return true;
 	}