[C++] Error for missing endif

A fix for https://github.com/google/kati/issues/55
diff --git a/parser.cc b/parser.cc
index 4b4012e..96b8ba3 100644
--- a/parser.cc
+++ b/parser.cc
@@ -87,6 +87,9 @@
 
       l_ = e + 1;
     }
+
+    if (!if_stack_.empty())
+      ERROR("%s:%d: *** missing `endif'.", loc_.filename, loc_.lineno + 1);
   }
 
   static void Init() {