Fortran: character length of pointer assignments in structure constructors

gcc/fortran/ChangeLog:

	PR fortran/50549
	* resolve.cc (resolve_structure_cons): Reject pointer assignments
	of character with different lengths in structure constructor.

gcc/testsuite/ChangeLog:

	PR fortran/50549
	* gfortran.dg/char_pointer_assign_7.f90: New test.