Delete VAO in Mesh only if it is nonzero.

glDeleteVertexArrays() was called even after the Mesh was moved out to
another object. On my NVidia it was working but it might cause issues
elsewhere.

src/Mesh.cpp

@@ -230,7 +230,7 @@ void Mesh::destroyImplementationDefault() {}
 void Mesh::destroyImplementationVAO() {
     /** @todo Get some extension wrangler instead to avoid linker errors to glDeleteVertexArrays() on ES2 */
     #ifndef MAGNUM_TARGET_GLES2
-    glDeleteVertexArrays(1, &vao);
+    if(vao) glDeleteVertexArrays(1, &vao);
     #endif
 }