Mark `mrb->backtrace.exc` as GC root; fix
@@ -857,6 +857,7 @@ root_scan_phase(mrb_state *mrb, mrb_gc *gc)
mrb_gc_mark(mrb, (struct RBasic*)mrb->top_self);
/* mark exception */
mrb_gc_mark(mrb, (struct RBasic*)mrb->exc);
+ mrb_gc_mark(mrb, (struct RBasic*)mrb->backtrace.exc);
/* mark pre-allocated exception */
mrb_gc_mark(mrb, (struct RBasic*)mrb->nomem_err);
#ifdef MRB_GC_FIXED_ARENA
