Fix issue with reading zero bytes in Gzip_read.

pg_dump expects a read request of zero bytes to be a no-op; see for
example ReadStr().  Gzip_read got this wrong and falsely supposed
that the resulting gzret == 0 indicated an error.  We could complicate
that error-checking logic some more, but it seems best to just fall
out immediately when passed size == 0.

This bug breaks the nominally-supported case of manually gzip'ing
the toc.dat file within a directory-style dump, so back-patch to v16
where this code came in.  (Prior branches already have a short-circuit
for size == 0 before their only gzread call.)

Author: Tom Lane <[email protected]>
Reviewed-by: Chao Li <[email protected]>
Discussion: https://postgr.es/m/[email protected]
Backpatch-through: 16

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/1518b7d76aadcbdffa6214555b82b995e7404b38

Modified Files
--------------
src/bin/pg_dump/compress_gzip.c | 4 ++++
1 file changed, 4 insertions(+)

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