fix a bug when there is no uv map

Author: gaoxing

src/core/src/c3mx.cpp

@@ -467,7 +467,10 @@ namespace seed
 
 CTMexporter ctm;
 ctm.DefineMesh(aVertices.data(), aVertices.size() / 3, aIndices.data(), aIndices.size() / 3, aNormals.data());
-ctm.AddUVMap(aUVCoords.data(), nullptr, nullptr);
+if (aUVCoords.size())
+{
+ctm.AddUVMap(aUVCoords.data(), nullptr, nullptr);
+}
 ctm.SaveCustom(_ctm_write_buf, &bufferData);
 return bb;
 }