revert own change: don't use memchr for strnlen, causes problems when the len is longer then the string data, instead use strnlen from freebsd.
also simplify empty string checks in logic_ops.c


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55345 954f8c5b-7b00-dc11-b283-0030488c597c