Bugfix: Uploading of partialy uploaded Geometry

When Geometry is uploaded, one can add new VertexStream,
which would cause a Runtime error during next upload (null reference)

src/alternativa/engine3d/resources/Geometry.as

@@ -652,10 +652,12 @@ package alternativa.engine3d.resources {
 				_indexBuffer = null;
 				for (i = 0; i < numBuffers; i++) {
 					vBuffer = _vertexStreams[i];
+					if (vBuffer.buffer != null) {
 						vBuffer.buffer.dispose();
 						vBuffer.buffer = null;
 					}
 				}
+			}
 			if (_indices.length <= 0 || _numVertices <= 0) {
 				return;
 			}