The mColors[0] array was allocated with m_numColors elements but
indexed up to mNumVertices (= numFaces * 3), causing an out-of-bounds
write when the color count is less than the vertex count.
Allocate mColors[0] with mNumVertices (matching mNormals and
mTextureCoords), and add a bounds check on idx before reading
from the source color array.
Fixes#6468
Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>
b4c7912d21