Safer assert_all_finite.

Check for Inf/NaN when X.sum() is not finite. This avoids the risk of
false positives because of sum overflow while remaining performant for
the usual case where all values are finite.

This idea is taken from:

   https://github.com/tecki/numpy/commit/57167f7c02b02bfb49204233eaee0f289ad37c92