Parsing: replace partial with total function

Calling `tail` on an empty list raises an exception, while calling the
otherwise equivalent `drop 1` will return the empty list again.

src/Text/Pandoc/Parsing.hs

@@ -1380,5 +1380,5 @@ insertIncludedFile blocks dirs f = do
   bs <- blocks
   setInput oldInput
   setPosition oldPos
-  updateState $ \s -> s{ stateContainers = tail $ stateContainers s }
+  updateState $ \s -> s{ stateContainers = drop 1 $ stateContainers s }
   return bs