sv_dup(): avoid cloning empty arrays

sv_dup(): avoid cloning empty arrays

Testing the ARRAY pointer is insufficent.  Arrays emptied by 'shift' or
'pop' may still have non-NULL 'ARRAY' pointers.  Check more carefully to
determine whether the array has anything worth duplicating.  If not, reset
the FILL and MAX offsets to -1 just as 'undef @ARRAY' would.

This avoids potential corruption in the PL_ptr_table during perl_clone().