io.c: fix 7bit coderange condition

* io.c (io_getc): fix 7bit coderange condition, check if ascii read
  data instead of read length. [ruby-core:55444] [Bug #8516]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41250 b2dd03c8-39d4-4d8f-98ff-823fe69b080e