Fix IndexError when shebang is just '#!/usr/bin/env'

[harrymander]

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[asottile]

is there an actual case where this came up? feels a bit like garbage-in-garbage-out

[harrymander]

I did come across it randomly due to a typo in a file. I agree that it is a pretty unlikely scenario, though it seems to be the only case where the library raises an exception due to the contents of the file. I suppose it's not too hard for client code to just check for this edge case if it may be a problem