MINOR: mux-h2/traces: add a missing trace on negative initial window size

MINOR: mux-h2/traces: add a missing trace on negative initial window size

When a negative initial windows size is reported, we're going to close
the connection, so it's important to report a trace to explain why!
This should be backported at least to 3.1 and possibly 3.0 (adapting the
context since there's no glitches there).

(cherry picked from commit 86823c828f983bd986b150542c7a6482d60b291d)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit 35bb79b797b85f8b89c87b8b925bc50f6f25865c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>