fix Mul.is_zero

>>> from sympy import *
>>> oo.is_bounded
False
>>> oo*0
nan
>>> _.is_zero
>>> var('z', nonzero=False)
z
>>> z.is_zero
True
>>> (oo*z).is_zero  # this is wrong
True
>>> (oo*z)._eval_is_zero()  # but this is right
use oo instead of ub in tests