BUG/MEDIUM: mux-h2: do not quit the demux loop before setting END_REACHED

BUG/MEDIUM: mux-h2: do not quit the demux loop before setting END_REACHED

The demux loop could quit on missing data but the H2_CF_END_REACHED flag
would not be set in this case. This fixes a remaining situation where
previous commit f09612289 ("BUG/MEDIUM: mux-h2: handle remaining read0
cases") could not be sufficient and still leave CLOSE_WAIT. It's harder
to reproduce but was still observed in prod.

Now we quit via the end of the loop which already takes care of shutr.

This should be backported along with the patch above as far as 2.0.