Fix NPE for some (at least Mekanism) pipes

Happens when you try connecting some types of pipes (tested with Mekanism pipes) - NPEs and crashes upon chunk loading WHEN the smeltery was not finished.

(Crash log here: http://nopaste.info/f0940b534e.html)
I have rather created a direct fix than just opening an issue since we had to fix it on a live server ASAP.