dc crashes because a Blk* sometimes ends getting double freed.
To make it crash, any of these lines will do:
(each line is a separate input to dc):
1 sa 2 :a le d sa v :a
1 sa 2 :a le d sa :a
1 sa 2 :a le d sa c
Fix by assigning p to sptr->val before EMTPY causes a jump.
Additionally, dcgetwd() can return 0. all other uses check for
0 ptr; Also fix a buffer overflow.
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