Subtract with overflow panic in shell_expand

Reproduction:

fn test_func a0 a1 a2 a3
    echo $a0 $a1 $a2 $a3
end

let x y = "" ""
let z = $(test_func $x $y $x $x)

Build information:

rustc -V: rustc 1.33.0-nightly (2fadb0a16 2019-01-13)

git rev-parse HEAD: e40e7947

Stacktrace:

Stacktrace

thread 'main' panicked at 'attempt to subtract with overflow', src/lib/parser/shell_expand/mod.rs:79:33
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
             at src/libcore/panicking.rs:49
   9: ion_shell::parser::shell_expand::expand_process
             at src/lib/parser/shell_expand/mod.rs:79
  10: ion_shell::parser::shell_expand::expand_single_string_token
             at src/lib/parser/shell_expand/mod.rs:528
  11: ion_shell::parser::shell_expand::expand_tokens
             at src/lib/parser/shell_expand/mod.rs:603
  12: ion_shell::parser::shell_expand::expand_string
             at src/lib/parser/shell_expand/mod.rs:260
  13: ion_shell::parser::assignments::checker::get_string
             at src/lib/parser/assignments/checker.rs:147
  14: ion_shell::parser::assignments::checker::value_check
             at src/lib/parser/assignments/checker.rs:245
  15: ::local
             at src/lib/shell/assignments.rs:285
  16: ::execute_statement
             at src/lib/shell/flow.rs:181
  17: ::on_command
             at src/lib/shell/flow.rs:491
  18: ion_shell::shell::binary::terminate::terminate_script_quotes
             at ./src/lib/shell/binary/terminate.rs:37
  19: ::terminate_script_quotes
             at ./src/lib/shell/binary/mod.rs:165
  20: ion_shell::shell::Shell::execute_script
             at ./src/lib/shell/mod.rs:207
  21: ion::main
             at src/main.rs:54
  22: std::rt::lang_start::{{closure}}
             at /rustc/2fadb0a16c8737a45746e95df9138912590ed8ad/src/libstd/rt.rs:64
  23: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:297
  24: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:92
  25: std::rt::lang_start_internal
             at src/libstd/panicking.rs:276
             at src/libstd/panic.rs:388
             at src/libstd/rt.rs:48
  26: std::rt::lang_start
             at /rustc/2fadb0a16c8737a45746e95df9138912590ed8ad/src/libstd/rt.rs:64
  27: main
  28: __libc_start_main
  29: _start