BUG/MEDIUM: h2: properly handle the END_STREAM flag on empty DATA frames

BUG/MEDIUM: h2: properly handle the END_STREAM flag on empty DATA frames

Peter Lindegaard Hansen reported a problem affecting some POST requests
sent by MSIE on 1.8.3. Lukas found that we incorrectly dealt with the
END_STREAM flag on empty DATA frames.

What happens in fact is that while we correctly report that we've read a
zero-byte frame, since commit 8fc016d ("BUG/MEDIUM: h2: support uploading
partial DATA frames") backported into 1.8.2, we've been able to return
without updating the parser's state nor checking the frame flags in this
case.

The fix is trival, we just need not to return too early.

This fix must be backported to 1.8.