fix: null-guard Add_Engine_Ref for vertex/index buffers

[seer-by-sentry]

This PR addresses CLIENT-510 by adding null checks before calling Add_Engine_Ref() for render_state.vertex_buffers[0] in DX8Wrapper::Set_Vertex_Buffer and render_state.index_buffer in DX8Wrapper::Set_Index_Buffer.

Previously, Add_Engine_Ref() was called unconditionally, leading to an EXCEPTION_ACCESS_VIOLATION_WRITE when the vertex or index buffer pointer was NULL (e.g., for zero-vertex meshes or failed dynamic allocations). The corresponding Release_Engine_Ref() calls in these functions were already null-guarded, making this an inconsistent omission.

The fix ensures that Add_Engine_Ref() is only called if the buffer pointer is valid, preventing the access violation.

Fixes CLIENT-510

_{This PR was automatically generated by Sentry. You can adjust this setting at any time.}

[github-actions]

⚠️ Title/Commit Validation Failed

Invalid PR title:

• fix: null-guard Add_Engine_Ref for vertex/index buffers

Invalid commit messages:

• fix: null-guard Add_Engine_Ref for vertex/index buffers
  PR titles and commit messages must follow conventional commits format:

type: Description
type(scope): Description

Allowed types: bugfix, build, chore, ci, docs, fix, feat, perf, refactor, revert, style, test, tweak, unify

See CONTRIBUTING.md for details.