Assert that backreferences array is not freed rather than handling it

Assert that backreferences array is not freed rather than handling it

In several places where the weakrefs backreferences array is used
or freed, the code checks whether the array has already been freed
and if so skips. Since the array already being freed is a bad bug,
lets instead assert that this never happens, based on the
assumptions that (a) such premature freeing bugs are likely ironed
out by now, (b) if they aren't then we want to know about them and
fix them rather than silently skip.

p4raw-id: //depot/perl@34210