stefcal improper channel selection memory guzzler

[IanHeywood]

I guess this deserves the bug tag, but it's as much a careless user bug as a software bug.

I'm selecting channels via ms_sel.ms_channel_end and ms_sel.ms_channel_start and running (phase only) stefcal via mqt.run. If I select an end channel that's higher than the number of the final channel in the MS then the memory usage just runs away into swapsville and I have to kill the process. Equal sized chunks further down the band for the same MS only use a couple of GB.