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
	---
	 string.c                 | 98 +++++++++++++++++++++++++++++++-----------------
	 test/ruby/test_string.rb | 11 ++++++
	 2 files changed, 75 insertions(+), 34 deletions(-)