Panicked at 'byte index 1 is out of bounds on invalid query

[alamb]

Describe the bug
@syheliel reports the following errors:

1. arrow-datafusion/datafusion/physical-expr/src/[planner.rs:54](http://planner.rs:54/)
      if &variable_names[0][0..2] == "@@" {
2. arrow-datafusion/datafusion/physical-expr/src/[planner.rs:1740](http://planner.rs:1740/)
      if &var_names[0][0..1] == "@" {

Which will panic if the length of variable names is less than 2 characters.

These bugs can be fixed by using get(0..2) instead of [0..2] for a slice.

To Reproduce
Here is one reproducer:

❯ SELECT  N | "".   LT;
thread 'main' panicked at 'byte index 1 is out of bounds of ``', library/core/src/str/mod.rs:107:9
stack backtrace:
   0: rust_begin_unwind
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/std/src/panicking.rs:584:5
   1: core::panicking::panic_fmt
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/panicking.rs:142:14
   2: core::str::slice_error_fail_rt
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/str/mod.rs:107:9
   3: core::ops::function::FnOnce::call_once
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/ops/function.rs:248:5
   4: core::intrinsics::const_eval_select
             at /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/core/src/intrinsics.rs:2376:5
   5: core::str::slice_error_fail
...

Expected behavior
A valid error message rather than panic

Additional context
This is similar to #3137 / fixed #3162

[alamb]

This is probably a good first issue as the change is identified and there is a test case