Panic on empty strings

Created by: xTibor

Reproduction: Running "" (empty string) in Ion panics instead of giving a "command not found" error. Panics both on Linux and Redox.

Build information: rustc -V: rustc 1.19.0-nightly (04145943a 2017-06-19) git rev-parse HEAD: 94e5e8e9

Misc: Stacktrace: https://gist.github.com/xTibor/05328391daced5b21bafaebe37b24c71