split was leaving PL_sv_undef in unused ary slots

split was leaving PL_sv_undef in unused ary slots

This:

    @a = split(/-/,"-");
    $a[1] = undef;
    $a[0] = 0;

was giving

    Modification of a read-only value attempted at foo line 3.

This is because:

1) unused slots in AvARRAY between AvFILL and AvMAX should always be
null; av_clear(), av_extend() etc do this; while av_store(), if storing
to a slot N somewhere between AvFILL and AvMAX, doesn't bother to clear
between (AvFILL+1)..(N-1) on the assumption that everyone else plays
nicely.

2) pp_split() when splitting directly to an array, sometimes over-splits
and has to null out the excess elements;

3) Since perl 5.19.4, unused AV slots are now marked with NULL rather than
&PL_sv_undef;

4) pp_split was still using &PL_sv_undef;

The fault was with (4), and is easily fixed.