This should improve performance and improve readability.

This first prints the number into a temporary buffer and then copies the
number to the output.

A positive side effect is that cJSON no longer reserves more space for
the number in the output than is necessary.

Because sizeof('\0') is actually sizeof(int) not sizeof(char).

Thanks @mmkeeper

This could only happen if the maximum SIZE_T is not at least 2 times
bigger than INT_MAX. Not sure if this can happen on real systems, but
better be safe then sorry.

This fixes -Wimplicit-fallthrough warnings with GCC7.