fix undefined behavior in count_mmac_params

[sorokin]

When compiled -fsanitize=undefined nasm produced this error message:

asm/preproc.c:2523:25: runtime error: member access within null pointer of type 'struct Token'

The problem is reproducible on tests avx512f, avx512cd, avx512pf and avx512er in the test suite.

The problematic line was:

    /* Advance to the next comma */
    maybe_comma = &t->next;                            <<< HERE
    while (tok_isnt(t, ',')) {
        if (!tok_white(t))
            comma = NULL; /* Non-empty parameter */
        maybe_comma = &t->next;
        t = t->next;
    }

When t is NULL this line doesn't cause memory access, but it is still an undefined behavior according to C standard.

I believe that the underlying problem is that this loop doesn't have a sound invariant about maybe_comma:

• On first iteration: *maybe_comma == t->next
• On the following iterations: *maybe_comma == t

I don't know what the intended loop invariant is and I decided to just mechanically fix the deferencing of NULL pointer, completely preserving the existing behavior.