BUG/MAJOR: mux-h2: Report a protocol error for any DATA frame before headers

BUG/MAJOR: mux-h2: Report a protocol error for any DATA frame before headers

If any DATA frame is received before all headers are fully received, a
protocol error must be reported. It is required by the HTTP/2 RFC but it is
also important because the HTTP analyzers expect the first HTX block is a
start-line. It leads to a crash if this statement is not respected.

For instance, it is possible to trigger a crash by sending an interim
message with a DATA frame (It may be an empty DATA frame with the ES
flag). AFAIK, only the server side is affected by this bug.

To fix the issue, an protocol error is reported for the stream.

This patch should fix the issue #2291. It must be backported as far as 2.2
(and probably to 2.0 too).