fuse: handle zero sized retrieve correctly

Dereferencing req->page_descs[0] will Oops if req->max_pages is zero.

Reported-by:  <syzbot+c1e36d30ee3416289cc0@syzkaller.appspotmail.com>
Tested-by:  <syzbot+c1e36d30ee3416289cc0@syzkaller.appspotmail.com>
Fixes: b2430d75 ("fuse: add per-page descriptor <offset, length> to fuse_req")
Cc: <stable@vger.kernel.org> # v3.9
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>