bus error in eval.c

I found a bus error while debugging a pathological case between 1.6 and 
1.8 that makes my code run 100x slower in 1.8. I think I have the 
proper patch for it. WOOT. I just found the minimally reproducible test 
case:

	class Hash
	  alias :old_inspect :inspect
	  def inspect
	    raise "no"
	  end
	end

	eval "{}.blah"

I've gotten this to work on ruby 1.8.1 (from cvs) on both macosx/darwin 
and freebsd.

gdb shows:

> Program received signal EXC_BAD_ACCESS, Could not access memory.
> eval (self=5880980, src=5880500, scope=4, file=0x87798 "(eval)", 
> line=1) at eval.c:6052
> 6052                        rb_str_update(mesg, 0, 0, 
> RARRAY(errat)->ptr[0]);
>
> (gdb) bt
> #0  eval (self=5880980, src=5880500, scope=4, file=0x87798 "(eval)", 
> line=1) at eval.c:6052
> #1  0x0000f6f8 in rb_f_eval (argc=-1073872096, argv=0x4065c2, 
> self=5880980) at eval.c:6118
> #2  0x0000dbd8 in rb_call0 (klass=1936544, recv=5880980, id=3905, 
> oid=3905, argc=1, argv=0xbffe0820, body=0x1d7288, nosuper=0) at 
> eval.c:5429
> #3  0x0000e518 in rb_call (klass=1936544, recv=5880980, mid=3905, 
> argc=1, argv=0xbffe0820, scope=1) at eval.c:5650
> # ...
> (gdb) p errat
> $1 = 4

ruby.h says that Qnil = 4. Here is my patch:

*** eval.c.~1.616.2.14.~	Wed Mar  3 11:01:01 2004
--- eval.c	Tue Mar  9 20:29:01 2004
***************
*** 6049,6055 ****
   		mesg  = rb_attr_get(ruby_errinfo, rb_intern("mesg"));
   		if (!NIL_P(mesg) && TYPE(mesg) == T_STRING) {
   		    rb_str_update(mesg, 0, 0, rb_str_new2(": "));
! 		    rb_str_update(mesg, 0, 0, RARRAY(errat)->ptr[0]);
   		}
   		RARRAY(errat)->ptr[0] = RARRAY(backtrace(-2))->ptr[0];
   	    }
--- 6049,6057 ----
   		mesg  = rb_attr_get(ruby_errinfo, rb_intern("mesg"));
   		if (!NIL_P(mesg) && TYPE(mesg) == T_STRING) {
   		    rb_str_update(mesg, 0, 0, rb_str_new2(": "));
! 		    if (!NIL_P(errat) && TYPE(errat) == T_ARRAY) {
! 		        rb_str_update(mesg, 0, 0, RARRAY(errat)->ptr[0]);
! 		    }
   		}
   		RARRAY(errat)->ptr[0] = RARRAY(backtrace(-2))->ptr[0];
   	    }

I could also see both conditionals being merged...