Prefer memcpy when exact sizes have been calculated as this removes the
implication that the string might be smaller than the length argument.
Further, passing in `len + 1` to BLI_strncpy without clamping by the
destination buffer size is reads like a common mistake,
where the length of the source may exceed the destination buffer size.
While using `std::min(sizeof(dst), len + 1)` would avoid the confusion
it's complicating a statement which can use memcpy instead.
bdd189f593