Bugfix/issue 900 check pos finite nan

#### Summary:
Fix check_pos_definite (issue #900).

#### Intended Effect:
Make sure check_pos_definite fails when NaN is passed in. Before, if any nans are present in the matrix, the check will still pass instead of throwing.

#### How to Verify:
Run test/unit/math/error_handling/matrix/check_pos_definite

#### Side Effects:
None.

#### Documentation:
None.

#### Reviewer Suggestions:
Anyone.