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stacktrace: protect against gc/stack shuffling

--- a/src/sl.c

+++ b/src/sl.c

@@ -909,15 +909,16 @@

 static sl_v

 _stacktrace(usize top)

 {

-	sl_v lst = sl_nil, v = sl_nil;

+	sl_v lst = sl_nil, v = sl_nil, fn = sl_nil;

 	sl_gc_handle(&lst);

 	sl_gc_handle(&v);

+	sl_gc_handle(&fn);

 	while(top > 0){

 		const u8int *ip1 = (void*)sl.stack[top-1];

 		int sz = sl.stack[top-2]+1;

-		sl_v *bp = sl.stack+top-4-sz;

-		sl_v fn = bp[0];

+		usize bp = top-4-sz;

+		fn = sl.stack[bp+0];

 		v = alloc_vec(sz+1, 0);

 		if(iscbuiltin(fn))

 			vec_elt(v, 0) = lst == sl_nil ? mk_ptr(sl.cpc) : fn;

@@ -928,7 +929,7 @@

 		}

 		vec_elt(v, 1) = fn;

 		for(int i = 1; i < sz; i++){

-			sl_v si = bp[i];

+			sl_v si = sl.stack[bp+i];

 			// if there's an error evaluating argument defaults some slots

 			// might be left set to sl_unbound

 			vec_elt(v, i+1) = si == sl_unbound ? sl_void : si;

@@ -936,7 +937,7 @@

 		lst = mk_cons(v, lst);

 		top = sl.stack[top-3];

 	}

-	sl_free_gc_handles(2);

+	sl_free_gc_handles(3);

 	return lst;

 }