string.c: raise at invalid byte sequence

* string.c (rb_str_count): raise at invalid byte sequence argument
  even if single-byte optimization is effective.
  [ruby-dev:48442] [Bug #10078]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46897 b2dd03c8-39d4-4d8f-98ff-823fe69b080e