Fix memory leak in grapheme clusters

[Bug #20150]

String#grapheme_cluters and String#each_grapheme_cluster leaks memory
because if the string is not UTF-8, then the created regex will not
be freed.

For example:

    str = "hello world".encode(Encoding::UTF_32LE)

    10.times do
      1_000.times do
        str.grapheme_clusters
      end

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

Before:

    26000
    42256
    59008
    75792
    92528
    109232
    125936
    142672
    159392
    176160

After:

    9264
    9504
    9808
    10000
    10128
    10224
    10352
    10544
    10704
    10896

0 files changed, 0 insertions, 0 deletions