BUG/MEDIUM: mux-h2: Don't report an error on shutr if a shutw is pending

BUG/MEDIUM: mux-h2: Don't report an error on shutr if a shutw is pending

If a shutw is blocked because the mux is full or busy, we must defer the
shutr. In this case, the H2 stream is not in H2_SS_CLOSED state because the
shutw is also deferred. If the shutr is performed, this will lead to a
error.

Concretly, when the mux is unblocked, a RST_STREAM is sent while in some
cases, an empty DATA frame with ES flag set could be sent.

This patch should be backported to all stable versions.