[perl #132225] segmentation fault while concurrently updating SetHash

[via RT]

# New Ticket Created by  David Lowe 
# Please include the string:  [perl #132225]
# in the subject line of all future correspondence about this issue. 
# <URL: https://rt.perl.org/Ticket/Display.html?id=132225 >


This short program crashes reliably (with a segmentation fault) on my
system:

```
#!/usr/bin/env perl6

use v6.c;

my $lock = Lock.new;
my $set  = SetHash.new;
await (^12).map: {
   start {
      for (^1000) {
         $lock.protect: { $set<1> = True  }
         $lock.protect: { $set<1> = False }
      }
   }
}
```

More information:

```
$ perl6 --version
This is Rakudo version 2017.09 built on MoarVM version 2017.09.1
implementing Perl 6.c.
```