codegen.c (mrb_last_insn): no previous instruction on top.

mrbgems/mruby-compiler/core/codegen.c

@@ -467,6 +467,10 @@ mrb_prev_pc(codegen_scope *s, const mrb_code *pc)
 static struct mrb_insn_data
 mrb_last_insn(codegen_scope *s)
 {
+  if (s->pc == 0) {
+    struct mrb_insn_data data = { OP_NOP, 0 };
+    return data;
+  }
   return mrb_decode_insn(&s->iseq[s->lastpc]);
 }