compress/flate: fix another deflate Reset inconsistency

While investigating #34121, fixed by CL 193605,
I discovered another case where Reset was not quite
resetting enough.

This specific case is not a problem in Reset itself but
rather that the Huffman bit writer in one code path
is using uninitialized memory left over from a previous
block, making the compression not choose the optimal
compression method.

Fixes #34121.

Change-Id: I29245b28214d924e382f91e2c56b4b8a9b7da13d
Reviewed-on: https://go-review.googlesource.com/c/go/+/243140


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>