Clean up recent Coverity complaints.

Clean up recent Coverity complaints.

Commit 5c649fe15 introduced a memory leak into pg_basebackup's
parse_compress_options.  (I simplified nearby code while at it.)

Commit 9a974cbcb introduced a memory leak into pg_dump's
binary_upgrade_set_pg_class_oids.

Coverity also complained about a call of SnapBuildProcessChange that
ignored the result, unlike every other call of that function.  This
is evidently intentional, so add a (void) cast to indicate that.
(It's also old, dating to b89e15105; I suppose the reason it showed
up now is 7a5f6b474's recent rearrangement of nearby code.)