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merge revision(s) r45468: [Backport #9687]

    * gc.c (gc_before_sweep): cap `malloc_limit' to
      gc_params.malloc_limit_max.  It can grow and grow with such case:
        `loop{"a" * (1024 ** 2)}'
      [Bug #9687]
      This issue is pointed by Tim Robertson.
      http://www.omniref.com/blog/blog/2014/03/27/ruby-garbage-collection-still-not-ready-for-production/

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@45817 b2dd03c8-39d4-4d8f-98ff-823fe69b080e