Fix missing return in SceneUtils::InstantiateMesh error path

[rootkiller6788]

Summary

SceneUtils::InstantiateMesh checks that the scene has a base mesh matching the requested instance.mesh_index, but the constructed error Status was never returned:

if (scene.NumMeshes() <= instance.mesh_index.value()) {
  Status(Status::DRACO_ERROR, "Scene has no corresponding base mesh.");  // discarded
}

The bounds check was therefore a no-op and execution continued to scene.GetMesh(instance.mesh_index), which performs an unchecked meshes_[index] access and crashes on an out-of-range index.

This fix returns the error status as the surrounding code clearly intends, and adds a regression test that instantiates a mesh with an index not present in the scene.

Verification

Built draco_tests (MinGW, DRACO_TRANSCODER_SUPPORTED=ON):

• Without the fix, the new SceneUtilsTest.TestInstantiateMeshWithInvalidMeshIndex test segfaults (exit code 139) due to the out-of-bounds access.
• With the fix, the test passes and InstantiateMesh returns an error status.

The existing TestInstantiateMesh and TestInstantiateMeshWithIdentityTransformation tests are unaffected.