Implement a `KVT_Validate()` function to check if `s_kvt*` item is valid

[LexouDuck]

Like the title says, this would be a very useful function to have.

Furthermore, it should be called at the beginning of each complex recursive function:

• KVT_Duplicate()
• KVT_Merge()
• KVT_Get()
• [JSON/TOML]_Print()
• [JSON/TOML]_ToString()