[clangd] Add a missing "return nullptr;" to the SelectionTree::commonAncestor.

This would fix some mysterious crashes on codeAction in clangd.

llvm-svn: 365751

clang-tools-extra/clangd/Selection.cpp

@@ -364,6 +364,7 @@ const Node *SelectionTree::commonAncestor() const {
     // The tree only contains ancestors of the interesting nodes.
     assert(!Ancestor->Children.empty() && "bad node in selection tree");
   }
+  return nullptr;
 }
 
 } // namespace clangd