Fix an integer underflow bug in libotp
    
    Temporarily storing the offset time in an unsigned integer causes the
    value of the offset to underflow when a (valid) negative offset value
    is generated. Using a signed variable avoids this problem.
    
    https://fedorahosted.org/freeipa/ticket/5333
    
    Reviewed-By: Tomas Babej <tbabej@redhat.com>