BUG/MEDIUM: mux-h2: unbreak receipt of large DATA frames

BUG/MEDIUM: mux-h2: unbreak receipt of large DATA frames

Recent optimization in commit 4d7a88482 ("MEDIUM: mux-h2: don't try to
read more than needed") broke the receipt of large DATA frames because
it would unconditionally subscribe if there was some room left, thus
preventing any new rx from being done since subscription may only be
done once the end was reached, as indicated by ret == 0.

However, fixing this uncovered that in HTX mode previous versions might
occasionally be affected as well, when an available frame is the same
size as the maximum data that may fit into an HTX buffer, we may end
up reading that whole frame and still subscribe since it's still allowed
to receive, thus causing issues to read the next frame.

This patch will only work for 2.1-dev but a minor adaptation will be
needed for earlier versions (down to 1.9, where subscribe() was added).