vm_exec.h: fix CHECK_VM_STACK_OVERFLOW_FOR_INSN

* vm_exec.h (CHECK_VM_STACK_OVERFLOW_FOR_INSN): surround with
  do/while (0), and remove unnecessary casts.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42196 b2dd03c8-39d4-4d8f-98ff-823fe69b080e