compiler: avoid crashing on bad self-referential type

    
    The compiler was crashing partway through emitting an error for a bad
    self-referential struct type (which refers to one of its own fields
    via an unsafe.Offset expression). Tweak the offset logic to check for
    this class of type error and exit cleanly.
    
    Fixes golang/go#24180
    
    Reviewed-on: https://go-review.googlesource.com/97855

From-SVN: r258112

gcc/go/gofrontend/MERGE

-821960465883fbdd96568f2325f55ee4b05de1cb
+262d629b1592f681fef396166a671e46cdb31230
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

gcc/go/gofrontend/expressions.cc

@@ -8224,6 +8224,11 @@ Builtin_call_expression::do_numeric_constant_value(Numeric_constant* nc) const
             return false;
           if (st->named_type() != NULL)
             st->named_type()->convert(this->gogo_);
+          if (st->is_error_type())
+            {
+              go_assert(saw_errors());
+              return false;
+            }
           int64_t offset;
           this->seen_ = true;
           bool ok = st->struct_type()->backend_field_offset(this->gogo_,