Fortran: fix associate with assumed-length character array [PR115700]

gcc/fortran/ChangeLog:

	PR fortran/115700
	* trans-stmt.cc (trans_associate_var): When the associate target
	is an array-valued character variable, the length is known at entry
	of the associate block.  Move setting of string length of the
	selector to the initialization part of the block.

gcc/testsuite/ChangeLog:

	PR fortran/115700
	* gfortran.dg/associate_69.f90: New test.