BUG/MEDIUM: quic: fix invalid conn reject with CONNECTION_REFUSED

BUG/MEDIUM: quic: fix invalid conn reject with CONNECTION_REFUSED

quic-initial rules were implemented just recently. For some actions, a
new flags field was added in quic_dgram structure. This is used to
report the result of the rules execution.

However, this flags field was left uninitialized. Depending on its
value, it may close the connection to be wrongly rejected via
CONNECTION_REFUSED. Fix this by properly set flags value to 0.

No need to backport.