gltfpack: Only enable sparse mode for non-empty indices

Point clouds use empty indices, so indices.size() would be less
than the vertex count. The actual code that would execute then is a no-op,
as it would leave sparse empty, but if the mechanism of enabling sparse
mode changes in the future this may regress, so add a condition to be
explicit.

Author: zeux

gltf/parsegltf.cpp

@@ -216,7 +216,7 @@ static void parseMeshesGltf(cgltf_data* data, std::vector<Mesh>& meshes, std::ve
 			std::vector<unsigned int> sparse;
 
 			// if the index data is very sparse, switch to deindexing on the fly to avoid the excessive cost of reading large accessors
-			if (result.indices.size() < vertex_count / 2)
+			if (!result.indices.empty() && result.indices.size() < vertex_count / 2)
 			{
 				sparse = result.indices;