keep-lines doesn't properly convert rend to a marker

[Paul Du Bois]

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In GNU Emacs 21.2.1 (i386-msvc-nt5.1.2600)
 of 2002-03-19 on buffy
configured using `configure --with-msvc (12.00)'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ENU
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: nil

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

If one calls keep-lines and passes an integer (not a marker!) in rend, rend
will not be converted to a marker within keep-lines.  This causes problems
if any deletions are made.

Recent input:
w C-z M-o C-u C-f C-u C-f C-h f <return> <help-echo> 
<help-echo> <down-mouse-2> <mouse-2> <mouse-wheel> 
<mouse-wheel> <mouse-wheel> <mouse-wheel> <mouse-wheel> 
<mouse-wheel> <mouse-wheel> <mouse-wheel> <mouse-wheel> 
<mouse-wheel> <mouse-wheel> <mouse-wheel> <mouse-wheel> 
<mouse-wheel> <mouse-wheel> <mouse-wheel> <mouse-wheel> 
<mouse-wheel> <mouse-wheel> <mouse-wheel> <mouse-wheel> 
<mouse-wheel> <mouse-wheel> <switch-frame> <down-mouse-1> 
<mouse-movement> <mouse-movement> <mouse-movement> 
<mouse-movement> <mouse-movement> <mouse-movement> 
<mouse-movement> <mouse-movement> <mouse-movement> 
<mouse-movement> <mouse-1> M-x s u b m i t C-w b u 
g C-g M-x a p r <return> b u g <return> M-o C-s b u 
g - r e p o C-s C-s C-a M-x r p e <backspace> <backspace> 
e p o r t - e m <tab> <return>

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