vim-patch:7.4.312

Problem:    Cannot figure out what argument list is being used for a window.
Solution:   Add the arglistid() function. (Marcin Szamotulski)

https://code.google.com/p/vim/source/detail?r=v7-4-312
In the original patch `*tp` is initialized with `NULL`. As there is a path through the following code that does not change the variable it should be kept that way.