Do not memcpy from empty source

[yangchi]

memcpy(dst, src, size) is a Undefined behavior if src is nullptr, even if size is 0. (https://en.cppreference.com/w/c/string/byte/memcpy)
I ran into a case where the input Stack parameter for the scanner serialize call has the has NULL content, 0 size and 0 capacity. And that caused runtime crash when build with UBSAN.

I thought about checking src being nullptr instead of size being 0. But I think if we ever get an input where src is nullptr but size > 0, that's bug somewhere else and we should crash program to surface that. Nullptr + zero size sounds relatively legit to actually hit this function, we just shouldn't try to do memcpy with such empty src. Thus i went with checking the size, not the src ptr. Let me know if people prefer other way. Thanks.

[yangchi]

@fwcd could you please take a look? Thank you!