openmp, fortran: Fix ICE when the procedure name cannot be found in declare variant...

openmp, fortran: Fix ICE when the procedure name cannot be found in declare variant directives [PR104428]

The result of searching for the procedure name symbol should be checked in
case the symbol cannot be found to avoid a null dereference.

gcc/fortran/

	PR fortran/104428
	* trans-openmp.cc (gfc_trans_omp_declare_variant): Check that proc_st
	is non-NULL before dereferencing.  Add line number to error message.

gcc/testsuite/

	PR fortran/104428
	* gfortran.dg/gomp/pr104428.f90: New.