BUG/MEDIUM: mux-h2: Check the number of headers in HEADERS frame after decoding

BUG/MEDIUM: mux-h2: Check the number of headers in HEADERS frame after decoding

There is no explicit test on the number of headers when a HEADERS frame is
received. It is implicitely limited by the size of the header list. But it
is twice the configured limit to be sure to decode the frame.

So now, a check is performed after the HTX message was created. This way, we
are sure to not exceed the configured limit after the decoding stage. If
there are too many headers, a parsing error is reported.

Note the same is performed on the trailers.

This patch should patially address the issue #2685. It should be backported
to all stable versions.