`calc` builtin has strange behavior when a parenthesis is the last character

Created by: huntergoldstein

Reproduction: Attempt to use the command:

calc "5 * 6 - (7 ** 3)"

Expected behavior:

$ calc "5 * 6 - (7 ** 3)"
-313

Actual behavior:

$ calc "5 * 6 - (7 ** 3)"
thread '<unnamed>' panicked at 'byte index 20 is out of bounds of `"5 * 6 - (7 ** 3)"`', /checkout/src/libcore/str/mod.rs:2133
note: Run with `RUST_BACKTRACE=1` for a backtrace

$ calc "5 * 6 - (4 + 4)"
<calc waits for input>

Build information:

• rustc -V: rustc 1.20.0-nightly (445077963 2017-06-20)
• git rev-parse HEAD: f16d4aef1bde530e47e956cf2c28f775a15a7b39

Blocking / Related: #243 (closed)