Don't abort pg_basebackup when receiving empty WAL block

This is a similar fix as c6ec879 9.2. This should never happen in 9.3 and newer since the special case cannot happen there, but this patch synchronizes up the code so there is no confusion on why they're different. An empty block is as harmless in 9.3 as it was in 9.2, and can safely be ignored.

Author: mhagander

src/bin/pg_basebackup/receivelog.c

@@ -989,7 +989,7 @@ HandleCopyStream(PGconn *conn, XLogRecPtr startpos, uint32 timeline,
 hdr_len += 8;/* dataStart */
 hdr_len += 8;/* walEnd */
 hdr_len += 8;/* sendTime */
-if (r < hdr_len + 1)
+if (r < hdr_len)
 {
 fprintf(stderr, _("%s: streaming header too small: %d\n"),
 progname, r);