Questionable behavior of the normally unused instruction OP_DEBUG

[dearblue]

I just followed the code by eye, so if I'm wrong or that's the intended behavior, please ignore it.

1. two consecutive FETCH_XXX() from iseq may

   While OP_DEBUG is defined as operand type BBB in include/mruby/ops.h, it is defined as operand type Z in src/vm.c followed by FETCH_BBB().
   I assume that OP_DEBUG always intends to FETCH_BBB(), but if the operand is extended with instruction OP_EXT3, for example, FETCH_SSB() is performed followed by FETCH_BBB().

   If this is not the intention, it is better to change OP_DEBUG in src/vm.c to BBB or OP_DEBUG in include/mruby/ops.h to Z.

2. inconsistency when the configuration macro MRB_USE_DEBUG_HOOK is defined

   In normal instructions, mrb->debug_op_hook() is called before decoding the operand.
   For OP_DEBUG, mrb->debug_op_hook() is called again after FETCH_BBB(). In addition, NULL check of mrb->debug_op_hook is not performed at this time.