Fix finalization of decompressor astreamers.

Fix finalization of decompressor astreamers.

Send the correct amount of data to the next astreamer, not the
whole allocated buffer size.  This bug escaped detection because
in present uses the next astreamer is always a tar-file parser
which is insensitive to trailing garbage.  But that may not
be true in future uses.

Author: Andrew Dunstan <andrew@dunslane.net>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2178517.1774064942@sss.pgh.pa.us
Backpatch-through: 15