pg_dump: Fix gzip compression of empty data

pg_dump: Fix gzip compression of empty data

The pg_dump Compressor API has three basic callbacks - Allocate, Write
and End.  The gzip implementation (since e9960732a) wrongly assumed the
Write function would always be called, and deferred the initialization
of the internal compression system until the first such call.  But when
there's no data to compress (e.g. for empty LO), this would result in
not finalizing the compression state (because it was not actually
initialized), producing invalid dump.

Fixed by initializing the internal compression system in the Allocate
call, whenever the caller provides the Write.  For decompression the
state is not needed, so we leave the private_data member unpopulated.

Introduces a pg_dump TAP test compressing an empty large object.

This also rearranges the functions to their original order, to make
diffs against older code simpler to understand.  Finally, replace an
unreachable pg_fatal() with a simple assert check.

Reported-by: Justin Pryzby
Author: Justin Pryzby, Georgios Kokolatos
Reviewed-by: Georgios Kokolatos, Tomas Vondra
https://postgr.es/m/20230228235834.GC30529%40telsasoft.com