BUG/MEDIUM: checks: do not dereference a list as a tcpcheck struct

BUG/MEDIUM: checks: do not dereference a list as a tcpcheck struct

The method used to skip to next rule in the list is wrong, it assumes
that the list element starts at the same offset as the rule. It happens
to be true on most architectures since the list is the first element for
now but it's definitely wrong. Now the code doesn't crash anymore when
the struct list is moved anywhere else in the struct tcpcheck_rule.

This fix must be backported to 1.5.