Intervals crash

[Chong Yidong]

I'm getting a crash due to your recent EMACS_UINT change.

  emacs -Q
  C-x C-f tr TAB => crash

  #0  abort () at emacs.c:423
  #1  0x0824356b in delete_interval (i=0x84a8cbc) at intervals.c:1269
  #2  0x08243a36 in merge_interval_right (i=0x84a8cbc) at intervals.c:1474
  #3  0x08242d93 in adjust_intervals_for_insertion (tree=0x84b7e7c, position=12,
      length=8) at intervals.c:1027
  #4  0x0824396d in offset_intervals (buffer=0x85de228, start=12, length=8)
      at intervals.c:1436
  #5  0x08191207 in insert_from_string_1 (string=141199641, pos=0, pos_byte=0,
      nchars=8, nbytes=8, inherit=0, before_markers=0) at insdel.c:1087
  #6  0x08190ddf in insert_from_string (string=141199641, pos=0, pos_byte=0,
      length=8, length_byte=8, inherit=0) at insdel.c:986
  ...
  Lisp Backtrace:
  "completion--replace" (0xbfffd654)
  "completion--do-completion" (0xbfffd944)
  "minibuffer-complete" (0xbfffdc54)
  ...

The LENGTH macro in intervals.h:114 has to be able to return a negative
number.  This is probably worth reverting, until you come up with a
proper fix.