result_of::at<Seq, N>::type is't defined when sizeof of sequence is less than N, close #6507, #7651

Original proposals suggest making as a hard error (by MPL asssert /
static assert), however this change just doesn't define due to
SFINAE-friendly.
Merge pull request #41 from Flast/patch-2

`result_of::at<Seq, N>::type` is't defined when sizeof of sequence is less than N