Fix lighting when lightpos and spec coef are 0

Zero to the power of zero is technically undefined, NVIDIA chokes but AMD seems to accept it as 1.0.  Have not tested on Android.

Fixes #2424.

Would use isnan() but seems unavailable on our targets.

http://www.askamathematician.com/2010/12/q-what-does-00-zero-raised-to-the-zeroth-power-equal-why-do-mathematicians-and-high-school-teachers-disagree/

-[Unknown]