BUG/MINOR: qpack: reject invalid increment count decoding

BUG/MINOR: qpack: reject invalid increment count decoding

Close the connection using QPACK_DECODER_STREAM_ERROR when receiving an
invalid insert count increment. As haproxy does not use dynamic table,
this instruction must never be emitted by the peer.

Prior to this patch, haproxy silently ignored such instruction which is
not conform to the QUIC specification.

This should be backported up to 2.6. Note that on 2.6 qcc_set_error()
must be replaced by function qcc_emit_cc_app().