BUG/MINOR: quic: initialize msg_flags before sendmsg

BUG/MINOR: quic: initialize msg_flags before sendmsg

Previously, msghdr struct used for sendmsg was memset to 0. This was
updated for performance reason with each members individually defined.
This is done by the following commit :

  commit 107d6d75465419a09d90c790edb617091a04468a
  OPTIM: quic: improve slightly qc_snd_buf() internal

msg_flags is the only member unset, as sendmsg manual page reports that
it is unused. However, this caused a coverity report. In the end, it is
better to explicitely set it to 0 to avoid any future interrogations,
compiler warning or even portability issues.

This should fix coverity report from github issue #2455.

No need to backport unless above patch is.