compressor shall check that CRC-STATIC changes upon updated static context

[Didier Barvaux]

The note from §5.9.2 of RFC 3095 is not implemented yet:

   Note: Whenever the CRC-STATIC fields change, the new saved CRC state
   after 1) is compared with the old state. If the states are
   identical, the CRC cannot catch the error consisting in the
   decompressor not having updated the static context. In U/O-mode the
   compressor SHOULD then for a while use packet types with another CRC
   length, for which there is a difference in CRC state, to ensure error
   detection.