net/mgen: Fix build with Clang 6

../common/mgenSocketList.cpp:231:20: error: cannot initialize return object of type 'MgenSocketList::Item *' with an rvalue of type 'bool'

return false;
       ^~~~~

http://beefy11.nyi.freebsd.org/data/head-i386-default/p477696_s338122/logs/errors/mgen-4.2b6.log