dc: fix crashes with : operator (thanks istvan bak)

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.

0 files changed, 0 insertions, 0 deletions