[ruby/json] Fix memory leak when exception is raised during JSON generation part 2

Commit https://github.com/ruby/json/commit/44df509dc2de fixed it for StandardError, but other exceptions and
jumps are also possible. Use rb_ensure() to release FBuffer instead of
rb_rescue().

A reproducer:

    o = Object.new
    def o.to_json(a) = throw :a
    a = ["make heap allocation"*100, o]

    10.times do
      100_000.times do
        catch(:a) { JSON(a) }
      end

      puts `ps -o rss= -p #{$$}`
    end

https://github.com/ruby/json/commit/9b7b648ecd

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