Fix memory leak in xlogreader facility.
Fix memory leak in xlogreader facility.

XLogReaderFree failed to free the per-block data buffers, when they
happened to not be used by the latest read WAL record.

Michael Paquier. Backpatch to 9.5, where the per-block buffers were added.

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl