Bug#178402: cdebconf/slang: do not accept multiple selections with multiselect

[Petter Reinholdtsen <pere@hungry.com>]

I got the following backtrace after installaing and attaching gdb to
the running frontend:

  Program received signal SIGSEGV, Segmentation fault.
  0x40017c45 in strvacat () from /usr/lib/libdebconf.so.1
  (gdb) bt
  #0 0x40017c45 in strvacat () from /usr/lib/libdebconf.so.1
  #1 0x0000000b in ?? ()
  (gdb)

I tried to look at the source (slang.c), but did not see anything
obviously wrong.