fixed https://bugs.launchpad.net/panda3d/+bug/383969 , whas doing default_curve-> when default_curve is null, now just returns false

panda/src/parametrics/parametricCurveCollection.cxx

@@ -518,6 +518,10 @@ evaluate(float t, LVecBase3f &xyz, LVecBase3f &hpr) const {
     }
   }
 
+  if (default_curve == (ParametricCurve *)NULL) {
+    return false;
+  }
+
   if (xyz_curve == (ParametricCurve *)NULL) {
     xyz_curve = default_curve;
   }