BUG/MAJOR: h2: reset the parser's state on mux buffer full

BUG/MAJOR: h2: reset the parser's state on mux buffer full

The h2 parser has this specificity that if it cannot send the headers
frame resulting from the headers it just parsed, it needs to drop it
and parse it again later. Since commit 8852850 ("MEDIUM: h1: let the
caller pass the initial parser's state"), when this happens the parser
remains in the data state and the headers are not parsed again next
time, resulting in a parse error. Let's reset the parser on exit there.

No backport is needed.