[ruby/psych] Guard from memory leak in Psych::Emitter#start_document

When an exception is raised, it can leak memory in `head`. There are two
places that can leak memory:

1. `Check_Type(tuple, T_ARRAY)` can leak memory if `tuple` is not an
   array.
2. `StringValue(name)` and `StringValue(value)` if they are not strings
   and the call to `to_str` does not return a string.

This commit fixes these memory leaks by wrapping the code around a
rb_ensure so that the memory is freed in all cases.

The following code demonstrates the memory leak:

    emitter = Psych::Emitter.new(StringIO.new)
    nil_to_string_tags = [[nil, "tag:TALOS"]] + ([1] * 1000)
    expected_array_tags = [1] * 1000

    10.times do
      1_000.times do
        # Raises `no implicit conversion of nil into String`
        emitter.start_document([], nil_to_string_tags, 0)
      rescue TypeError
      end

      1_000.times do
        # Raises `wrong argument type Integer (expected Array)`
        emitter.start_document([], expected_array_tags, 0)
      rescue TypeError
      end

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

Before:

    47248
    79728
    111968
    144224
    176480
    208896
    241104
    273280
    305472
    337664

After:

    14832
    15088
    15344
    15344
    15360
    15632
    15632
    15632
    15648
    15648

https://github.com/ruby/psych/commit/053af73818

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