cmd/go: don't assume decimal in test_fuzz_mutate_crash
In the float test in test_fuzz_mutate_crash, don't assume the mutator
will generate a decimal during mutation. The probability it will is
quite high, but it is not guaranteed, which can cause a flake. Since we
are not really testing that the mutator will do this kind of mutation,
just that a mutation happens, just check that the input is not the zero
value like the rest of the targets.