fix segfault from splitting 1hp long worm

fix segfault from splitting 1hp long worm

     Fix the crash From a bug report, where
having the hit that cuts a long worm into two also take the original down
to 1 HP would result in clone_mon() returning null and nethack crashing due
to a segmentation fault or access violation.  The same thing could happen
if there's been enough long worms created to get them flagged as extinct.

     This bug was only present in 3.4.3.  Prior to that, cut_worm() did
its own monster creation inline instead of calling clone_mon(), ignoring
extinction and too-low hit points.