Use goto fail for error handling and fix some memory leaks on the way

[FSMaxB]

This replaces many different exit paths out of some functions with only one in case of errors.

This way it is much easier to reason about the code and properly manage allocated resources (and it actually fixes some memory leaks).

This relates to #56

[benlachman]

Careful with goto fail. It has a song written about it's ability to bite you: https://www.youtube.com/watch?v=tQms037U72w

[FSMaxB]

@benlachman I'm fully aware of the potential problems with goto fail.

The problem that Apple had in it's SSL library cannot happen in cJSON because we always use curly braces in if conditions.

There is the saying that goto is harmful, but it is not harmful in general, because there are about two legitimate use cases, one of which is goto fail.

The advantage of goto fail is that you can use a programming pattern where you allocate all resources at the beginning of a function and only ever free them in the fail section, this makes double frees and memory leaks much less likely. (and in this case, there have been several memory leaks).

If you add a new allocation at the beginning of a function, you really don't want to update every single if condition that returns from the function and add the call to free to it.

[benlachman]

@FSMaxB: Good. I just wanted to put it out there since it does have a history. Curly braces around all conditionals and loops are the way to go. 👍🏿