[perl #81750] Perl 5.12: undef-as-hashref bug

[perl #81750] Perl 5.12: undef-as-hashref bug

The addition of the boolkeys op type in commit 867fa1e2d did not
account for the fact that rv2hv (%{}) can sometimes return undef
(%$undef with strict refs turned off).

When the boolkeys op is created (and the rv2hv becomes its kid), the
rv2hv is flagged with OPf_REF, meaning that it must return a hash, not
the contents.

Perl_softrefxv in pp.c checks for that flag. If it is set, it dies
with ‘Can't use an undefined value as a HASH reference’ for unde-
fined values.

This commit changes it to make an exception if rv2hv->op_next is a
boolkeys op. It also changes pp_boolkeys to account for undef.