Ruby

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