[Bug #20453] segfault in Regexp timeout

	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)