The error check was added for `FileAccessUnix` but it's not an error when both
`p_src` and `p_length` are zero.
Added correct error checks to all implementations to prevent the actual
erroneous case: `p_src` is nullptr but `p_length > 0` (risk of null pointer
indexing).
Fixes#33564.
(cherry picked from commit 01d5c463be)