Add an assert.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@77527 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Index/Indexer.cpp b/lib/Index/Indexer.cpp
index 9a49496..0b534b3 100644
--- a/lib/Index/Indexer.cpp
+++ b/lib/Index/Indexer.cpp
@@ -38,6 +38,7 @@
 } // anonymous namespace
 
 void Indexer::IndexAST(TranslationUnit *TU) {
+  assert(TU && "Passed null TranslationUnit");
   EntityIndexer Idx(TU, Map);
   Prog.FindEntities(TU->getASTContext(), Idx);
 }