[perl #54384] [BUG] split opcode gives "failed assertion" when source string is null

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# New Ticket Created by  Patrick R. Michaud 
# Please include the string:  [perl #54384]
# in the subject line of all future correspondence about this issue. 
# <URL: http://rt.perl.org/rt3/Ticket/Display.html?id=54384 >

The split opcode gives "failed assertion 's'" when given a
null string argument.  Perhaps it should pretend it got the
null string, or at least throw a more "normal" exception.

Test case exposing bug:

$ cat y.pir
.sub main :main
    $S0 = 'hello world'
    $P0 = split ' ', $S0
    $I0 = elements $P0
    say $I0                   # 2

    null $S0
    $P0 = split ' ', $S0      # KABOOM
    $I0 = elements $P0
    say $I0
.end
$ ./parrot y.pir
2
src/string.c:802: failed assertion 's'
Backtrace - Obtained 16 stack frames (max trace depth is 32).
  (unknown)
    Parrot_confess
      string_length
        string_split
          Parrot_split_p_sc_s
[...lines deleted for brevity...]
Aborted
$

Pm