[perl #113904] [BUG] Null PMC Access when blocks get the wrong $_ sometimes in Rakudo

[Christian Bartolomaeus via RT]

This Null PMC Access error does not occur any longer. Also the SixModelObject 
error is gone. (Output of following commands is identical for Moar, Parrot and 
JVM.)

$ perl6 -e '$_ = 1; for ^2 -> $p { when $p { say "matched"; $_.say } }'
matched
1

$ perl6-m -e '$_ = 42; for 1 -> $p { if 1 { .say } }'
42

$ perl6 -e 'for 12 -> $a { if 1 { say $_.WHAT } }'
Nil

I extended the existing tests in S02-magicals/dollar-underscore.t a bit with 
the following commit: https://github.com/perl6/roast/commit/0e579ae7c7

I'm closing this ticket now.