conntrackd: build: fix crash when optional kernel modules are not loaded

Fix a possible crash if conntrackd sees DCCP, SCTP and ICMPv6 traffic
and the corresponding kernel modules that track this traffic are not
available.

Fixes: http://bugzilla.netfilter.org/show_bug.cgi?id=910
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>