Add RB_GC_GUARD for rb_str_to_parser_stringHEADmaster

I think this fixes the following random test failure that could not be
fixed for a long time:

```
  1) Failure:
TestSymbol#test_inspect_under_gc_compact_stress [/home/chkbuild/chkbuild/tmp/build/20240522T003003Z/ruby/test/ruby/test_symbol.rb:126]:
<":testing"> expected but was
<":\"\\x00\\x00\\x00\\x00\\x00\\x00\\x00\"">.
```

The value passed to this function is the return value of `rb_id2str`, so
it is never collected.  However, if auto_compact is enabled, the string
may move and `RSTRING_PTR(str)` became invalid.

This change prevents the string from being moved by RB_GC_GUARD.

0 files changed, 0 insertions, 0 deletions