fix undefined behavior in count_mmac_params

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.

Signed-off-by: Ivan Sorokin <vanyacpp@gmail.com>

asm/preproc.c

@@ -2813,6 +2813,9 @@ static Token **count_mmac_params(Token *tline, int *nparamp, Token ***paramsp)
                 }
             }
 
+            if (!t)
+                break;              /* End of string, no comma */
+
             /* Advance to the next comma */
             maybe_comma = &t->next;
             while (tok_isnt(t, ',')) {