* re.c (unescape_nonascii): cast -1 for the case char is unsigned char.
  If char is signed char, for example gcc for ARM or ppc64, it caused
  infinite loop.
  http://kmuto.jp/build-ruby/arm/ruby-trunk/log/20141106T013005Z.fail.html.gz
  http://rubyci.blob.core.windows.net/f19p8/ruby-trunk/log/20141106T090217Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48297 b2dd03c8-39d4-4d8f-98ff-823fe69b080e