Ignore velocity changes when the cloth solver does not converge.

This helps keep the simulation stable as long as there are only a few
substeps that become too constrained for the solver.

Eventually we need better feedback about these solver results, so that
artists can tweak situations specifically to resolve bad solver results.
This is somewhat similar to the camera tracker, which also can run into
cases that cannot be resolved and have to be fixed manually.

source/blender/blenkernel/intern/implicit.c

@@ -2430,8 +2430,10 @@ static bool simulate_implicit_euler(Implicit_Data *id, float dt)
 	// itend();
 	// printf("cg_filtered calc time: %f\n", (float)itval());
 
-	// advance velocities
-	add_lfvector_lfvector(id->Vnew, id->V, id->dV, numverts);
+	if (ok) {
+		// advance velocities
+		add_lfvector_lfvector(id->Vnew, id->V, id->dV, numverts);
+	}
 
 	del_lfvector(dFdXmV);