Initialize the error variable in uv_fs_copyfile() to zero. If
the source file being copied has no data in it, then it's
possible to make it to the exit without ever setting error.
In this situation, the function's behavior is unpredictable.
Refs: https://github.com/libuv/libuv/pull/1551
PR-URL: https://github.com/libuv/libuv/pull/1552
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>