BUG/MAJOR: mux_h2: Don't consume more payload than received for skipped frames

BUG/MAJOR: mux_h2: Don't consume more payload than received for skipped frames

When a frame is received for a unknown or already closed stream, it must be
skipped. This also happens when a stream error is reported. But we must be sure
to only skip received data. In the loop in h2_process_demux(), when such frames
are handled, all the frame lenght is systematically skipped. If the frame
payload is partially received, it leaves the demux buffer in an undefined
state. Because of this bug, all sort of errors may be observed, like crash or
intermittent freeze.

This patch must be backported to 2.0, 1.9 and 1.8.