BUG/MEDIUM: mux_h2: Add missing braces in h2_snd_buf()around trace+wakeup

BUG/MEDIUM: mux_h2: Add missing braces in h2_snd_buf()around trace+wakeup

This is a regression in 7838a79ba ("MEDIUM: mux-h2/trace: add lots of traces
all over the code"). The issue was found using -Wmisleading-indentation.
This patch fixes GitHub issue #1015.

The impact of this bug is that it could in theory cause occasional delays
on some long responses for connections having otherwise no traffic.

This patch should be backported to 2.1+, the commit was first tagged in
v2.1-dev2.

(cherry picked from commit 12a08d8849166ec0a779bbdee2ce9d768d5aa824)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>