Check for null return from getPowerReceiver

Buildcraft doesn't seem to ever return null but UniversalElectricity
does, so we'd throw NPEs.
Merge branch 'master' of https://github.com/OpenMods/OpenPeripheral
Adds writeSymbol to mystcraft desks

Mystcraft doesn't give us any success/failure info, sadly.
Merge remote-tracking branch 'upstream/master'