* array.c (rb_ary_count): check length to avoid SEGV
  while iterating. Remove other pointer loop when arg is given.
* test/ruby/test_array.rb (test_count): add test for bug.
  [ruby-core:56072] [Bug #8654]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42041 b2dd03c8-39d4-4d8f-98ff-823fe69b080e