Fix oflow bound in exp and pow.

Issue #600 highlighted a limit of current
implementation where early overflow is noticed,
which diverges from the C99 standard.

This patch adds test to catch this type of cases.

It only checks a few points therefore we don't
know for sure if overflow occurs at the right time
but we know if something is wrong in the overflow
region.

Fix both scalar and simd implementation (double precision)

Fixes #523.