[ruby-core:114224] [Ruby master Bug#19773] Intermittent SIGSEGV during marking of Ripper parser

Issue #19773 has been reported by eightbitraptor (Matthew Valentine-House).

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Bug #19773: Intermittent SIGSEGV during marking of Ripper parser
https://bugs.ruby-lang.org/issues/19773

* Author: eightbitraptor (Matthew Valentine-House)
* Status: Open
* Priority: Normal
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN
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Intermittent SIGSEGV during marking of the Ripper parser. A failing build [can be seen on ci.rvm.jp (build 4643684)](http://ci.rvm.jp/results/trunk-random2@ruby-sp2-docker/4643684).

Inside `ripper_s_allocate` first we use `TypedData_Make_Struct` to create a new `T_DATA` object in the Ruby heap. This allocates an empty struct (`struct ripper`). Further down the function we allocate another struct (`struct parser_params` during `rb_ruby_ripper_parser_allcoate`) and assign it to a pointer from the first struct, which is attached to the `T_DATA` object.

`rb_ruby_ripper_parser_allocate` can trigger GC, as it uses `ruby_xcalloc`. But if GC is triggered at this point then the `parser_params` have not been allocated yet, and so the `struct ripper` contains a `NULL` reference.

This causes a SEGV when we try and mark the `T_DATA` wrapper around the `struct ripper`.

This can be replicated locally using the following test script

```
require "ripper"
GC.stress = true
Ripper.new
```



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