Fix bogus tar-file padding logic for standby.signal.

Fix bogus tar-file padding logic for standby.signal.

When pg_basebackup -R is used, we inject standby.signal into the
tar file for the main tablespace. The proper thing to do is to pad
each file injected into the tar file out to a 512-byte boundary
by appending nulls, but here the file is of length 0 and we add
511 zero bytes.  Since 0 is already a multiple of 512, we should
not add any zero bytes. Do that instead.

Patch by me, reviewed by Tom Lane.

Discussion: http://postgr.es/m/CA+TgmobWbfReO9-XFk8urR1K4wTNwqoHx_v56t7=T8KaiEoKNw@mail.gmail.com