check for empty namespace

Prior to this commit it was possible to pop from an empty list,
generating an IndexError

cpp/ast.py

@@ -644,7 +644,7 @@ def generate(self):
                 self.namespaces.append(False)
                 continue
             if token.name == '}':
-                if self.namespaces.pop():
+                if self.namespaces and self.namespaces.pop():
                     self.namespace_stack.pop()
                 continue