bug#6187: Wrong type argument: number-of-marker-p, nil

[Christophe TROESTLER]

I've got the same error when I try to save a buffer for which I 
changed the coding system and that contains characters that cannot be 
encoded with the new encoding.

Here is my specific backtrace :

  min(nil)
  apply(min nil)
  select-safe-coding-system-interactively(1 3 (utf-8 viscii gb18030 
utf-7 utf-16 utf-16be-with-signature utf-16le-with-signature utf-16be 
utf-16le x-ctext iso-2022-7bit utf-8-auto utf-8-with-signature 
emacs-mule raw-text iso-2022-8bit-ss2 iso-2022-7bit-lock 
vietnamese-tcvn vietnamese-viqr vietnamese-vscii utf-7-imap 
utf-8-emacs no-conversion compound-text-with-extensions 
ctext-no-compositions iso-2022-7bit-ss2) (iso-latin-1-unix) nil utf-8)
  select-safe-coding-system(1 3 iso-latin-1-unix nil "/tmp/a.txt")
  write-region(nil nil "/tmp/a.txt" nil t "/tmp/a.txt")
  basic-save-buffer-2()
  basic-save-buffer-1()
  basic-save-buffer()
  save-buffer(1)
  call-interactively(save-buffer nil nil)