Invalid pointer returned if reallocate fails in ensure function

[timothyjohncarney]

https://github.com/DaveGamble/cJSON/blob/master/cJSON.c#L379

The calls to reallocate is assumed to return a valid pointer. If it fails, NULL should be returned instead of newbuffer + p->offset.

[FSMaxB]

This is definitely a bug and most likely also a security issue, since an attacker could make realloc fail at exactly the right moment while also providing a big JSON (big offset), thereby potentially producing a pointer to an arbitrary memory address that will be written to by cJSON later on.

I will fix it immediately and create a new bugfix release.

[FSMaxB]

Fixed in Release 1.5.7.