By switching from int to size_t in order to handle larger values,
r256322 introduced a bug that manifested itself on 32-bit
targets. Fixed by using the correct type to store the result of a
next_array_record call.

Regtested on x86_64-pc-linux-gnu and i686-pc-linux-gnu, committed to
trunk as obvious.

libgfortran/ChangeLog:

2018-01-08  Janne Blomqvist  <jb@gcc.gnu.org>

	PR 78534, bugfix for r256322
	* io/transfer.c (next_record_w): Use correct type for return value
	of next_array_record.

From-SVN: r256337