uv__fs_buf_iter: don't nullify req->bufs on EINTR

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
nodejs/node#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.

src/unix/fs.c

@@ -860,7 +860,11 @@ static ssize_t uv__fs_buf_iter(uv_fs_t* req, uv__fs_buf_iter_processor process)
 
   if (bufs != req->bufsml)
     uv__free(bufs);
-  req->bufs = NULL;
+
+  if (!(errno == EINTR && total == -1)) {
+    req->bufs = NULL;
+    req->nbufs = 0;
+  }
 
   return total;
 }