Fix bug #9139 with revisiting non-existent files.

Fix bug #9139 with revisiting non-existent files.

 src/fileio.c (Finsert_file_contents): If the file cannot be opened,
 set its "size" to -1.  This will set the modtime_size field of
 the corresponding buffer to -1, which is what
 verify-visited-file-modtime expects for files that do not exist.