[CCID]: Check if ccid is NULL in the hc_[tr]x_exit functions

For consistency with ccid_exit and to fix a bug when
IP_DCCP_UNLOAD_HACK is enabled as the control sock is not associated
to any CCID.

Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>