Ruby

cont.c: fix root fiber to_s

• cont.c (fiber_to_s): fix Fiber#to_s on root fibers which have no
  procs. [ruby-core:82629] [Bug #13859]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@59721 b2dd03c8-39d4-4d8f-98ff-823fe69b080e