rigidbody: Properly handle constrained objects not having rigid bodies

This is a pretty rare case that can be triggered by switching rigid body
and constraint groups before simulation was validated.
Code checked for existing physics objects but was missing else block.

source/blender/blenkernel/intern/rigidbody.c

@@ -635,6 +635,9 @@ void BKE_rigidbody_validate_sim_constraint(RigidBodyWorld *rbw, Object *ob, shor
 					break;
 			}
 		}
+		else { /* can't create constraint without both rigid bodies */
+			return;
+		}
 
 		RB_constraint_set_enabled(rbc->physics_constraint, rbc->flag & RBC_FLAG_ENABLED);