[clangd] Fix a compiler warning and test crashes caused in rL347038.

llvm-svn: 347039
diff --git a/clang-tools-extra/clangd/index/Background.cpp b/clang-tools-extra/clangd/index/Background.cpp
index da96f8c..bfb559f 100644
--- a/clang-tools-extra/clangd/index/Background.cpp
+++ b/clang-tools-extra/clangd/index/Background.cpp
@@ -43,7 +43,7 @@
       URISchemes(URISchemes),
       IndexStorageFactory(std::move(IndexStorageFactory)) {
   assert(ThreadPoolSize > 0 && "Thread pool size can't be zero.");
-  assert(IndexStorageFactory && "Storage factory can not be null!");
+  assert(this->IndexStorageFactory && "Storage factory can not be null!");
   while (ThreadPoolSize--) {
     ThreadPool.emplace_back([this] { run(); });
     // Set priority to low, since background indexing is a long running task we