linter: Assert that `_run()` is called with a buffer

yamllint/linter.py

@@ -94,6 +94,9 @@ def get_syntax_error(buffer):
 
 
 def _run(buffer, conf):
+    assert hasattr(buffer, '__getitem__'), \
+        '_run() argument must be a buffer, not a stream'
+
     # If the document contains a syntax error, save it and yield it at the
     # right line
     syntax_error = get_syntax_error(buffer)