Don't try to memcpy() to a NULL destination.

[icculus]

This function is called with a literal NULL for that pointer later in the file.

We had a user run into this in SDL; checking for NULL before memcpy()'ing resolved it.

[Youw]

This is a good one, thanks.
I wounder why Coverity didn't catch this.
It is clear (and I know it only now, when I checked explicitly for that) that it is an undefined behavior if either of src or dest of memcpy is a null pointer even of the size is 0.

[Youw]

We had a user run into this in SDL

I wounder what's your user's environment. Clearly that check wasn't required for years, otherwise someone else would hit it by now.
My guess that the implementation of C-runtime library on macOS wasn't as strict as The Standard states.

UPD: OK, I see a discussion under libsdl-org/SDL#8428 so this does look like a fix for a potential (theoretical) issue.
Good to have it anyway, thanks.