optional jacobian fix

gtsam/base/OptionalJacobian.h

@@ -94,8 +94,10 @@ public:
   /// Constructor that will resize a dynamic matrix (unless already correct)
   OptionalJacobian(Eigen::MatrixXd* dynamic) :
       map_(nullptr) {
-    dynamic->resize(Rows, Cols); // no malloc if correct size
-    usurp(dynamic->data());
+	if (dynamic) {
+      dynamic->resize(Rows, Cols); // no malloc if correct size
+      usurp(dynamic->data());
+	}
   }
 
   /**