Fixes Null References Error from ASCompletion.Context.ASContext->GetDeclarationAtLine()

Fixes Null References Error from ASCompletion.Context.ASContext->GetDeclarationAtLine() if ASCompletion.Context.ASContext->cFile == null

Before fix:
![](http://service.crazypanda.ru/v/clip2net/d/0/qNvq0bsfkN.png)

After fix:
![](http://service.crazypanda.ru/v/clip2net/Z/v/1XygGTzfg1.png)

test: https://github.com/SlavaRa/flashdevelop/blob/tests/tests/External/Plugins/ASCompletionTest/Context/ASContextTest.cs#L164-171