BUG/MINOR: quic: reject packet too short for HP decryption

BUG/MINOR: quic: reject packet too short for HP decryption

Header protection can only be performed on a packet of a minimal size.
There was already a check for this in qc_do_rm_hp() but it did not use
the correct value.

Fix this by using the correct minimal size which is 20 bytes starting
from the packet number offset. This is enough to decrypt 4 bytes (PN max
size) and 16 bytes of IV. If the packet is not big enough, it is
still silently discarded.

This must be backported up to 2.6.