Bug in deleting multiple screens

[Scott Dolim]

I found another bug in ncurses 5.7.  It has to do with the following sequence:

A = newterm(...)
B = newterm(...)
...
delscreen(A)
delscreen(B)

After this, the variable _nc_screen_chain should end up NULL again,
but it is not.  It is left pointing at the address where A used to be.
 Later, the following sequence:

C = newterm()
...
delscreen(C)

will crash because C's _next_screen is pointing at A but that memory
has since been reused for something else.

The problem appears to be in delink_screen(), in that the previous
screen's _next_screen pointer is not being updated correctly.  I've
included a tiny patch that fixes the issue.  (Of course, you may have
caught and fixed this already in 5.8, or possibly aren't using a
global _nc_screen_chain anymore.)

-- Scott

lib_set_term.c.patch
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