The attached patch was provided by Steve Kargl.  After exploring for
possible other checks I settled on leaving the patch intact.

Two existing test cases updated as different but sensible error messages
resulted.

Regression tested on main line.

OK to commit?

Regards,

Jerry

Author: Steve Kargl <kargl@gcc.gnu.org>
Date:   Mon Dec 26 14:07:04 2022 -0800

     Modify checks to avoid referencing NULL pointer.
     Update test cases with error messages that changed as a result.

     gcc/fortran/ChangeLog:

             * decl.cc (attr_decl1): Guard against NULL pointer.
             * parse.cc (match_deferred_characteristics): Include
BT_CLASS in check for
               derived being undefined.

     gcc/testsuite/ChangeLog:

             * gfortran.dg/class_result_4.f90: Update error message check.
             * gfortran.dg/pr85779_3.f90: Update error message check.