Check if directory NLog.dll is detected in actually exists (#1432)
Check if directory NLog.dll is detected in actually exists (#1432)

* Check if directory NLog.dll is detected in actually exists

Fixes crash when an assembly location is found that doesn't actually exist. 

I believe this happens when using in-memory or bundling loading of the library, as described at https://github.com/NLog/NLog/issues/736#issuecomment-216694452.


BrutalCode/NLog,luigiberrettini/NLog,bjornbouetsmith/NLog,MartinTherriault/NLog,bjornbouetsmith/NLog,luigiberrettini/NLog,ie-zero/NLog,hubo0831/NLog,MartinTherriault/NLog,snakefoot/NLog,NLog/NLog,littlesmilelove/NLog,nazim9214/NLog,AndreGleichner/NLog,nazim9214/NLog,kevindaub/NLog,kevindaub/NLog,ie-zero/NLog,BrutalCode/NLog,bhaeussermann/NLog,304NotModified/NLog,UgurAldanmaz/NLog,tetrodoxin/NLog,UgurAldanmaz/NLog,bhaeussermann/NLog,littlesmilelove/NLog,tetrodoxin/NLog,sean-gilliam/NLog,AndreGleichner/NLog,hubo0831/NLog