Null pointer fixes

[FSMaxB]

This fixes a lot of potential null pointer dereferences and some memory leaks.

My search for NULL pointer dereferences was triggered by #182.

I wrote a test bench to test almost all functions with NULL pointers as inputs, but stupid me accidentally deleted it, so I'm gonna have to do that from scratch ...

• test cJSON functions with NULL parameters
• fix NULL pointer dereferences in cJSON
• test cJSON_Utils functions with NULL parameters
• fix NULL pointer dereferences in cJSON_Utils