[clang-tidy] Fix use-after-free in UseNullptrCheck.

llvm-svn: 245524
diff --git a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.h b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.h
index 40b9093..4b33f1e 100644
--- a/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.h
+++ b/clang-tools-extra/clang-tidy/modernize/UseNullptrCheck.h
@@ -24,6 +24,7 @@
   void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
 
 private:
+  const std::string NullMacrosStr;
   SmallVector<StringRef, 1> NullMacros;
 };