Fix compiler div-by-0 error when it encountered 1.0/0.0

with division by 0.
Tricked the compiler to use volatile local variable zero instead, so it
does not know for sure whether or not it's going to be zero.

to cJSON_ParseWithOpts, then that will store the error pointer (if
parsing fails).

thanks to kolman and Irwan Djajadi on SourceForge.