BUG/MEDIUM: mux-h2: also restart demuxing when data are pending in demux

BUG/MEDIUM: mux-h2: also restart demuxing when data are pending in demux

Commit 082f559d3 ("BUG/MEDIUM: h2: restart demuxing after releasing
buffer space") tried to address a situation where transfers could stall
after a read, but the condition was not completely covered : some stalls
may still happen at end of stream because there's nothing anymore to
receive and the last data lie in the demux buffer. Thus we must also
consider this state as a valid condition to restart demuxing.

No backport is needed.