Fix: Compositor: Potential crash if no active context is found

This is related to #140381, where the symptom of the bug was a crash
caused by an undefined behavior. In that case, setting a valid active
viewer key was the proper fix. However,
`find_active_context_recursive()` could return `nullptr` in theory so
the same problem might occur in the future.

The commit resolves the undefined behavior by avoiding the
dereferencing of a null pointer.

Pull Request: https://projects.blender.org/blender/blender/pulls/141270

source/blender/nodes/intern/derived_node_tree.cc

@@ -350,7 +350,14 @@ const DTreeContext &DerivedNodeTree::active_context() const
     return root_context();
   }
 
-  return *find_active_context_recursive(&root_context());
+  const DTreeContext *found_context = find_active_context_recursive(&root_context());
+  if (found_context == nullptr) {
+    /* There should always be a valid active context. */
+    BLI_assert_unreachable();
+    return root_context();
+  }
+
+  return *found_context;
 }
 
 /* Each nested node group gets its own cluster. Just as node groups, clusters can be nested. */