[perl #114230] [BUG] Null PMC access when doing //= on an undefined attribute and then calling it in Rakudo

[Christian Bartolomaeus via RT]

The Null PMC access no longer occurs. On Moar, Parrot and JVM this now runs 
fine (and has identical output):

$ perl6 -e 'class C { has &!x; method f { say defined &!x; &!x //= {"ook!" }; 
say defined &!x; &!x() } }; C.new.f'
False
True

I added a test to S12-attributes/class.t with the following commit: 
https://github.com/perl6/roast/commit/1345d8bf7a

I'm closing this ticket now.