fortran: fix checking of CHARACTER lengths in array constructors [PR70231]

gcc/fortran/ChangeLog:

	PR fortran/70231
	* trans-array.cc (trans_array_constructor): In absence of a typespec,
	use string length determined by get_array_ctor_strlen() to reasonably
	initialize auxiliary variable for bounds-checking.

gcc/testsuite/ChangeLog:

	PR fortran/70231
	* gfortran.dg/bounds_check_fail_7.f90: New test.