BUG/MINOR: mux-h2: fix the condition to close a cs-less h2s on the backend

A typo was introduced in the following commit : 927b88ba0 ("BUG/MAJOR:
mux-h2: fix race condition between close on both ends") making the test
on h2s->cs never being done and h2c->cs being dereferenced without being
tested. This also confirms that this condition does not happen on this
side but better fix it right now to be safe.

This must be backported to 1.9.
1 file changed