MINOR: quic: Splice the frames which could not be added to packets

MINOR: quic: Splice the frames which could not be added to packets

When building packets to send, we build frames computing their sizes
to have more chance to be added to new packets. There are rare cases
where this packet coult not be built because of the congestion control
which may for instance prevent us from building a packet with padding
(retransmitted Initial packets). In such a case, the pre-built frames
were lost because added to the packet frame list but not move packet
to the packet number space they come from.

With this patch we add the frames to the packet only if it could be built
and move them back to the packet number space if not.