codegen.c: skip `ADDI/SUBI` for zero operand.

mrbgems/mruby-compiler/core/codegen.c

@@ -781,6 +781,7 @@ gen_addsub(codegen_scope *s, uint8_t op, uint16_t dst)
       /* OP_ADDI/OP_SUBI takes upto 16bits */
       if (n > INT16_MAX || n < INT16_MIN) goto normal;
       rewind_pc(s);
+      if (n == 0) return;
       if (n > 0) {
         if (op == OP_ADD) genop_2(s, OP_ADDI, dst, (uint16_t)n);
         else genop_2(s, OP_SUBI, dst, (uint16_t)n);