Fix crash when Transform Extend multiple NLA strips

`tdn` was being incremented even though it wasn't used, which led to
changes in memory outside the HEAP limits.

source/blender/editors/transform/transform_convert_nla.cc

@@ -621,13 +621,15 @@ static void createTransNlaData(bContext *C, TransInfo *t)
         if (tdn->handle == 2) {
           tdn += 2;
         }
-        else {
+        else if (tdn->handle) {
           tdn++;
         }
       }
     }
   }
 
+  BLI_assert(tdn <= (((TransDataNla *)tc->custom.type.data) + tc->data_len));
+
   /* cleanup temp list */
   ANIM_animdata_freelist(&anim_data);
 }