BUG/MINOR: h3: prevent hypothetical demux failure on int overflow

BUG/MINOR: h3: prevent hypothetical demux failure on int overflow

h3s stores the current demux frame type and length as a state info. It
should be big enough to store a QUIC variable-length integer which is
the maximum H3 frame type and size.

Without this patch, there is a risk of integer overflow if H3 frame size
is bigger than INT_MAX. This can typically causes demux state mismatch
and demux frame error. However, no occurence has been found yet of this
bug with the current implementation.

This should be backported up to 2.6.