testclangimport.cpp: fixed potential nullptr dereference in parse() (#2927)

test/testclangimport.cpp

@@ -127,6 +127,9 @@ private:
         Tokenizer tokenizer(&settings, this);
         std::istringstream istr(clang);
         clangimport::parseClangAstDump(&tokenizer, istr);
+        if (!tokenizer.tokens()) {
+        	return std::string();
+        }
         return tokenizer.tokens()->stringifyList(true, false, false, false, false);
     }