PR 83704 Use size_t in write_character

PR 83704 Use size_t in write_character

For printing long characters, we need to use size_t instead of int in
the argument to write_character.

Regtested on x86_64-pc-linux-gnu, approved in the PR, committed to
trunk.

libgfortran/ChangeLog:

2018-01-06  Dominique d'Humieres  <dominiq@lps.ens.fr>
	    Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/83704
	* io/write.c (write_character): Use size_t instead of int for
	length.

Co-Authored-By: Janne Blomqvist <jb@gcc.gnu.org>
From-SVN: r256313