BUG/MAJOR: mux-h2: don't destroy the stream on failed allocation in h2_snd_buf()

BUG/MAJOR: mux-h2: don't destroy the stream on failed allocation in h2_snd_buf()

In case we cannot allocate a stream ID for an outgoing stream, the stream
will be aborted. The problem is that we also release it and it will be
destroyed again by the application detecting the error, leading to a NULL
dereference in h2_shutr() and h2_shutw(). Let's only mark the error on the
CS and let the rest of the code handle the close.

This should be backported to 1.9.