Check if irep is NULL before print_backtrace()

According to the valgrind log attached to #3438, proc->body.irep
may be NULL in some cases.

src/backtrace.c

@@ -118,6 +118,7 @@ each_backtrace(mrb_state *mrb, mrb_int ciidx, mrb_code *pc0, each_backtrace_func
     if (MRB_PROC_CFUNC_P(ci->proc)) continue;
 
     irep = ci->proc->body.irep;
+    if (!irep) continue;
 
     if (mrb->c->cibase[i].err) {
       pc = mrb->c->cibase[i].err;