`FormatContext` stream accessors can cause undefined behavior from safe code

Affected versions of `stainless_ffmpeg` exposed several safe public methods on `FormatContext` that accepted a `stream_index` parameter and used it in unsafe pointer operations without checking whether the index was valid. These methods performed pointer arithmetic and dereferenced the resulting stream pointer. Safe callers could pass a negative or out-of-bounds `stream_index`, which could cause out-of-bounds pointer access and undefined behavior from safe Rust code.

The issue was fixed in version `0.6.0` by marking the affected stream accessor methods as `unsafe` and documenting that callers must ensure `stream_index` is a valid stream index.