vm_insnhelper.c: fix implicit splat keyword argument

* vm_insnhelper.c (vm_callee_setup_keyword_arg): adjust VM stack
  pointer to get rid of overwriting splat arguments by arguments
  for `to_hash` conversion.  [ruby-core:63593] [Bug #10016]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46775 b2dd03c8-39d4-4d8f-98ff-823fe69b080e