Add protection to GenericStack to avoid writing to unallocated stacks

[HeCoding180]

Add protection to GenericStack to avoid writing to unallocated stacks:

• Add GS_StackNotAllocated_Error to cJSON_GenericStack_Result_t.
• Allways check if length is greater than 0 before committing any action.
• Return GS_StackNotAllocated_Error if stack is not allocated.