Instead of allocating struct h2_sess out of the same workspace that is
used as our rxbuffer, we rather keep it on the stack in h2_new_session.

If the size of the initial h/2 request payload is close to half the size
of the workspace, we would end up exhausting the workspace in HTC_RxInit
after allocating our h2sess struct.

Spotted by Federico G. Schwindt

Fixes: #2619