Use memset for initializing EXI documents and fragments

### Describe the problem

Currently, the initialization of exiDoc and exiFrag structures is done by explicitly setting each field to its default value. This can be simplified and made more maintainable by using memset to zero-initialize the entire structure.

Benefits:

- Cleaner and shorter code
- Reduced risk of missing a field during initialization
- Easier to maintain when structures evolve

### Describe your solution

Suggestion:

Replace manual field-by-field initialization with memset(&exiDoc, 0, sizeof(exiDoc)); or memset(&exiFrag, 0, sizeof(exiFrag)); where appropriate.

### Additional context

_No response_