Cycles: Fix memory leak with session

We didn't check for sync object being already allocated.
In fact, we simply need to use proper function to free
session.

intern/cycles/blender/blender_session.cpp

@@ -54,6 +54,7 @@ BlenderSession::BlenderSession(BL::RenderEngine& b_engine,
                                BL::BlendData& b_data,
                                bool preview_osl)
 : session(NULL),
+  sync(NULL),
   b_engine(b_engine),
   b_userpref(b_userpref),
   b_data(b_data),
@@ -81,6 +82,7 @@ BlenderSession::BlenderSession(BL::RenderEngine& b_engine,
                                BL::RegionView3D& b_rv3d,
                                int width, int height)
 : session(NULL),
+  sync(NULL),
   b_engine(b_engine),
   b_userpref(b_userpref),
   b_data(b_data),
@@ -209,11 +211,8 @@ void BlenderSession::reset_session(BL::BlendData& b_data, BL::Depsgraph& b_depsg
 		/* if scene or session parameters changed, it's easier to simply re-create
 		 * them rather than trying to distinguish which settings need to be updated
 		 */
-
-		delete session;
-
+		free_session();
 		create_session();
-
 		return;
 	}