Fix ticket:72 : SIGFPE with no exceptions enabled

 code/float-trap.lisp::
  * In the default case, check FOP to see if it's defined.  This means
    it's a divide exception.

 code/x86-vm.lisp::
  * Also check to see if the offending instruction is a DIV or IDIV,
    which means we got an integer overflow.  Return the appropriate
    values in this case.