Possible gaps/garbage in the output of XLOG reader

While playing with xlogreader, I was lucky enough to see one of the many
record validations to fail. After having some fun with gdb, I found out that
in some cases the reader does not enforce enough data to be in state->readBuf
before copying into state->readRecordBuf starts. This should not happen if the
callback always reads XLOG_BLCKSZ bytes, but in fact only *reqLen* is the
mandatory size of the chunk delivered.

There are probably various ways to fix this problem. Attached is what I did in
my environment. I hit the problem on 9.4.1, but the patch seems to apply to
master too.

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at