[perl #119749] [BUG] Null PMC access when looping over SomeEnum.enums in a blockless BEGIN in Rakudo

[Christian Bartolomaeus via RT]

There is still a Null PMC access on Parrot. On Moar and JVM it runs fine:

$ perl6-p -e 'enum A (a=>3); BEGIN for A.enums { }'
===SORRY!===
Null PMC access in get_bool()

$ perl6-m -e 'enum A (a=>3); BEGIN for A.enums { }; say "alive"'
alive

$ perl6-j -e 'enum A (a=>3); BEGIN for A.enums { }; say "alive"'
alive

I added a test (fudged 'skip' for Parrot) to S04-phasers/begin.t with commit 
https://github.com/perl6/roast/commit/489ebb2e46.