Fix 'insert-file-contents' when REPLACE is non-nil

Fix 'insert-file-contents' when REPLACE is non-nil

* src/fileio.c (maybe_move_gap): New function to move the gap to
the end of a buffer, if it isn't there already.
(Finsert_file_contents): Call 'maybe_move_gap' before using
conversion_buffer's text as a C 'char' array.  (Bug#23659)
* src/coding.c (decode_eol): Compute the byte increment before
calling del_range_2, because the latter can invalidate the pointer
to buffer text.