`let` panics when there are empty array variables in the shell

The let builtin is panicking with an index out of bounds panic when there are empty array variables within the shell.

Reproduction:

let x = []
let

Build information:

rustc -V: rustc 1.33.0-nightly (4c2be9c97 2019-01-22)

git rev-parse HEAD: 32319d7b

Stacktrace:

Stacktrace

thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', src/lib/shell/assignments.rs:40:41
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:385
   6: rust_begin_unwind
             at src/libstd/panicking.rs:312
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::panicking::panic_bounds_check
             at src/libcore/panicking.rs:61
   9: ion_shell::shell::assignments::list_vars::print_array
             at src/lib/shell/assignments.rs:40
  10: ion_shell::shell::assignments::list_vars
             at src/lib/shell/assignments.rs:53
  11: ::local
             at src/lib/shell/assignments.rs:159
  12: ::execute_statement
             at src/lib/shell/flow.rs:181
  13: ::on_command
             at src/lib/shell/flow.rs:491
  14: ::execute_interactive
             at src/lib/shell/binary/mod.rs:120
  15: ion::main
             at src/main.rs:66
  16: std::rt::lang_start::{{closure}}
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libstd/rt.rs:64
  17: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:297
  18: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:92
  19: std::rt::lang_start_internal
             at src/libstd/panicking.rs:276
             at src/libstd/panic.rs:388
             at src/libstd/rt.rs:48
  20: std::rt::lang_start
             at /rustc/4c2be9c97fb60a01c545b8e8fa61e4247ae5c9b2/src/libstd/rt.rs:64
  21: main
  22: __libc_start_main
  23: _start