Fix UB: int64_t >> 64

[iacore]

In https://en.cppreference.com/w/cpp/language/operator_arithmetic, section "Bitwise shift operators"

In any case, if the value of the right operand is negative or is greater or equal to the number of bits in the promoted left operand, the behavior is undefined.

I handled the specific UB case.

[estraier]

0 is not negative. I'll modify the assertion to be "width >= 0 && width < sizeof(int64_t)".

[iacore]

when width is 0, ((sizeof(num) - width) * 8) is 64, which is UB

[estraier]

Good point. Then, I'll put assert(width > 0).