Ruby

thread.c: Thread.start with Symbol

• thread.c (thread_do_start): fix segfault at start with Symbol.
  proc created by Symbol#to_proc does not have environment unless
  using refinements. [ruby-core:80147] [Bug #13313]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57969 b2dd03c8-39d4-4d8f-98ff-823fe69b080e