PR 78534 Modify string copy to avoid -Wstringop-overflow warning

When the character length is changed from int to size_t the existing
algorithm causes a -Wstringop-overflow warning with -O1 on the
gfortran.dg/allocate_deferred_char_scalar_1.f03 testcase. This change
is committed separately from the character length size change in order
to make bisecting potential performance issues easier.

2017-01-02  Janne Blomqvist  <jb@gcc.gnu.org>

        PR fortran/78534
	* trans-expr.c (gfc_trans_string_copy): Rework string copy
	algorithm to avoid -Wstringop-overflow warning.

From-SVN: r244003