Avoid assertion violation in intervals.c

Avoid assertion violation in intervals.c

* src/intervals.c (delete_interval): Allow negative values of
LENGTH (i).  This happens when delete_interval is called from
set_intervals_multibyte_1, because the caller zeroes out the
total_length field of the interval to be deleted.  See
https://lists.gnu.org/archive/html/emacs-devel/2020-04/msg00131.html
for more details.  See also a related old discussion at
https://lists.gnu.org/archive/html/emacs-devel/2012-07/msg00399.html.