Found by coverity

Thanks @rrvirtual for the suggestion.

Makes type conversions explicit, if they alter a value

When comparing the type, it has to be masked with 0xFF because of
cJSON_IsConst and cJSON_IsReference.

NOTE: This can change the assembly slightly, in my case it reordered two
instructions. This is due to the change from:

    which = (10 * which) + *pointer++ - '0';

to

    which = (10 * which) + (*pointer++ - '0');

This means that after the change, the subtraction runs before the
addition instead of after that. That shouldn't change the behavior
though.