[Bug #20453] segfault in Regexp timeoutHEADmaster

https://bugs.ruby-lang.org/issues/20228 started freeing `stk_base` to
avoid a memory leak. But `stk_base` is sometimes stack allocated (using
`xalloca`), so the free only works if the regex stack has grown enough
to hit `stack_double` (which uses `xmalloc` and `xrealloc`).

To reproduce the problem on master and 3.3.1:

```ruby
Regexp.timeout = 0.001
/^(a*)x$/ =~ "a" * 1000000 + "x"'
```

Some details about this potential fix:

`stk_base == stk_alloc` on
[init](https://github.com/ruby/ruby/blob/dde99215f2bc60c22a00fc941ff7f714f011e920/regexec.c#L1153),
so if `stk_base != stk_alloc` we can be sure we called
[`stack_double`](https://github.com/ruby/ruby/blob/dde99215f2bc60c22a00fc941ff7f714f011e920/regexec.c#L1210)
and it's safe to free. It's also safe to free if we've
[saved](https://github.com/ruby/ruby/blob/dde99215f2bc60c22a00fc941ff7f714f011e920/regexec.c#L1187-L1189)
the stack to `msa->stack_p`, since we do the `stk_base != stk_alloc`
check before saving.

This matches the check we do inside
[`stack_double`](https://github.com/ruby/ruby/blob/dde99215f2bc60c22a00fc941ff7f714f011e920/regexec.c#L1221)

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