Unterminated brace panic on `[{ }]`

Created by: xTibor

Reproduction: user:~/git/ion# [{ }]

Expected behavior: A syntax error or some other kind of error message.

Actual behavior: A panic with the following message: thread 'main' panicked at 'ion: fatal error with syntax validation: unterminated brace', src/lib/parser/shell_expand/words/mod.rs:519:9 Full backtrace

Build information: rustc -V: rustc 1.26.0-nightly (2789b067d 2018-03-06) git rev-parse HEAD: 96d5b5d2 System: KDE neon 5.12 (Ubuntu 16.04 LTS)