[ruby-core:108234] [Ruby master Bug#18728] Segfault since ruby 3.0.4 when parsing JSON

Issue #18728 has been reported by fschwahn (Fabian Schwahn).

----------------------------------------
Bug #18728: Segfault since ruby 3.0.4 when parsing JSON
https://bugs.ruby-lang.org/issues/18728

* Author: fschwahn (Fabian Schwahn)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.4p208 (2022-04-12 revision 3fa771dded) [x86_64-darwin21]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
This is a regression since 3.0.4. When parsing a JSON with an empty string as a key ruby segfaults.

Reproduction steps:
```
require "json"
puts JSON.parse('{ "": "foo" }')
```

leads to

```
RSTRING_PTR is returning NULL!! SIGSEGV is highly expected to follow immediately. If you could reproduce, attach your debugger here, and look at the passed string.
/Users/fabian/.rbenv/versions/3.0.4/lib/ruby/gems/3.0.0/gems/json-2.6.1/lib/json/common.rb:216: [BUG] Segmentation fault at 0x0000000000000000
ruby 3.0.4p208 (2022-04-12 revision 3fa771dded) [x86_64-darwin21]

-- Crash Report log information --------------------------------------------
   See Crash Report log file under the one of following:                    
     * ~/Library/Logs/DiagnosticReports                                     
     * /Library/Logs/DiagnosticReports                                      
   for more details.                                                        
Don't forget to include the above Crash Report log file in bug reports.     

-- Control frame information -----------------------------------------------
c:0004 p:---- s:0017 e:000016 CFUNC  :parse
c:0003 p:0027 s:0013 e:000012 METHOD /Users/fabian/.rbenv/versions/3.0.4/lib/ruby/gems/3.0.0/gems/json-2.6.1/lib/json/common.rb:216
c:0002 p:0020 s:0007 e:000005 EVAL   test.rb:3 [FINISH]
c:0001 p:0000 s:0003 E:002000 (none) [FINISH]

-- Ruby level backtrace information ----------------------------------------
test.rb:3:in `<main>'
/Users/fabian/.rbenv/versions/3.0.4/lib/ruby/gems/3.0.0/gems/json-2.6.1/lib/json/common.rb:216:in `parse'
/Users/fabian/.rbenv/versions/3.0.4/lib/ruby/gems/3.0.0/gems/json-2.6.1/lib/json/common.rb:216:in `parse'
```

The same script completes in ruby 3.0.3 and prior.

I also opened an issue with the `json`-gem (https://github.com/flori/json/issues/495), but as it broke with the ruby update it seems to be a bug in ruby itself? 



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