Fix crash on error 33 in ternary operator

If one of the two values is a string literal and the other is a
non-array, the symbol associated with the first one will be NULL.
But the code checked if sym->name!=NULL rather than sym!=NULL,
hence the crash.

--------- test code --------

main() {
    new a, b;
    return (a != 0 ? b : "string");
}

----- end of test code -----