BUG/MEDIUM: h2: don't consider pending data on detach if connection is in error

BUG/MEDIUM: h2: don't consider pending data on detach if connection is in error

Interrupting an h2load test shows that some connections remain active till
the client timeout. This is due to the fact that h2_detach() immediately
returns if the h2s flags indicate that the h2s is still waiting for some
buffer room in the output mux (possibly to emit a response or to send some
window updates). If the connection is broken, these data will never leave
and must not prevent the stream from being terminated nor the connection
from being released.

This fix must be backported to 1.8.