[perl #56908] DBI memory leak in 5.10.0 due to change 26530

[perl #56908] DBI memory leak in 5.10.0 due to change 26530

A weakref to a HV would leak, because the xhv_backreferences
array is created with a refcount of 2 (to avoid premature freeing
during global destruction), but the RC was only decremented once
when the parent HV was freed.
Also, when thread cloned, the new array was being created with a
RC of 1, rather than 2, which coincidentally worked due to the
first bug.

p4raw-id: //depot/perl@34209