http2: fix session memory accounting after pausing

The ability to pause input processing was added in 8a4a1931 but
introduced a session memory accounting mismatch leading to potential
NGHTTP2_ENHANCE_YOUR_CALM errors.

After pausing
(https://github.com/nodejs/node/blob/f36331c1bfa4c4c202346b05dc3bd672f653e4df/src/node_http2.cc#L871),
the early return on line 873 skips the
DecrementCurrentSessionMemory(stream_buf_.len) call below (line 878).

When we later finished processing the input chunk
(https://github.com/nodejs/node/blob/f36331c1bfa4c4c202346b05dc3bd672f653e4df/src/node_http2.cc#L1858),
we were calling DecrementCurrentSessionMemory(stream_buf_offset_) [line
1875] which was a no-op since we just set stream_buf_offset_ to 0 [line
1873].

The correct amount to decrement by is still stream_buf_.len, since
that's the amount we skipped previously (line 878).

Fixes: https://github.com/nodejs/node/issues/29223
Refs: https://github.com/nodejs/node/commit/164ac5b241b96089e6bad5bb83ea416966b3245f

PR-URL: https://github.com/nodejs/node/pull/30684


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>