BUG/MEDIUM: h2: don't report an error after parsing a 100-continue response

BUG/MEDIUM: h2: don't report an error after parsing a 100-continue response

Yves Lafon reported a breakage with 100-continue. In fact the problem
is caused when an 1xx is the last response in the buffer (which commonly
is the case). We loop back immediately into the parser with what remains
of the input buffer (ie: nothing), while it is not expected to be called
with an empty response, so it fails.

Let's simply get back to the caller to decide whether or not more data
are expected to be sent.

This fix needs to be backported to 1.8.