panic when deal with `@` operator

[syheliel]

Describe the bug

thread 'main' panicked at 'byte index 2 is out of bounds of `@`', library/core/src/str/mod.rs:107:9

To Reproduce
main.rs

use async_std;
use datafusion;
use datafusion::prelude::*;
fn main() {

    let data = "SElECT @=   X#=?!~ 5";
    let a = async {
        let ctx = SessionContext::new();
        let df = ctx.sql(data).await;
    };
    async_std::task::block_on(a);
}

Cargo.toml

[package]
name = "crash-example"
version = "0.1.0"
edition = "2021"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html

[dependencies]
datafusion = "10"
async-std = "1"

Expected behavior
return an Error instead of panic.

[syheliel]

In src/execution/context.rs:1444-1448, the program doesn't check the length of variable_names[0](in my cause the value is "@") and lead to fetching invalid address for variable_names[0][0..2].

        let provider_type = if &variable_names[0][0..2] == "@@" {
            VarType::System
        } else {
            VarType::UserDefined
        };

[alamb]

Thanks @syheliel -- I'll take a look at this