[SPARK-19334][SQL] Fix the code injection vulnerability related to Generator functions.

## What changes were proposed in this pull request?

Similar to SPARK-15165, codegen is in danger of arbitrary code injection. The root cause is how variable names are created by codegen.
In GenerateExec#codeGenAccessor, a variable name is created like as follows.

```
val value = ctx.freshName(name)
```

The variable `value` is named based on the value of the variable `name` and the value of `name` is from schema given by users so an attacker can attack with queries like as follows.

```
SELECT inline(array(cast(struct(1) AS struct<`=new Object() { {f();} public void f() {throw new RuntimeException("This exception is injected.");} public int x;}.x`:int>)))
```

In the example above, a RuntimeException is thrown but an attacker can replace it with arbitrary code.

## How was this patch tested?

Added a new test case.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #16681 from sarutak/SPARK-19334.