uv__fs_buf_iter currently sets req->bufs to NULL after it is done, but
if the operation fails with EINTR then it will be retried, at which
point it expects the bufs to not be NULL, causing a seg fault as in
https://github.com/nodejs/node/issues/4291.
uv__fs_buf_iter should not set req->bufs to NULL if the operation
fails with EINTR.
Also, when it sets req->bufs to NULL, it should set req->nbufs to 0 as
well, so we don't have the messy situation of a positive nbufs with no
actual bufs.
PR-URL: https://github.com/libuv/libuv/pull/661
Reviewed-By: Fedor Indutny <fedor@indutny.com>
