BUG/MEDIUM: quic: Possible crashes when sending too short Initial packets

BUG/MEDIUM: quic: Possible crashes when sending too short Initial packets

This may happen during handshakes when Handshake packets cannot be coalesced
to a first Initial packet because of TX frame allocation failures (from
qc_build_frms()). This leads too short (not padded) Initial packets to be sent.
This is detected by a BUG_ON() in qc_send_ppkts().

To avoid this an Handshake packet without ack-eliciting frames which should have
been built by qc_build_frms() is built.

Must be backported as far as 2.6.