uncompress: Avoid reading an extra byte

When reading the next code in a stream, avoid reading an extra byte if
we're going to throw it away. When there's no more bits to extract from
the stream, bits will be 0 and we'll mask the read byte with 0 anyway.
At worst, this will avoid reading one past the end of gbuf array (which
is not possible in well formed streams).

PR: 127912
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D47041

(cherry picked from commit 818c7b769a4f7d3c8fecc4cf491f4e22ef816eba)