Fix memory leak in regexp grapheme clusters

	[Bug #20161]

	The cc->mbuf gets overwritten, so we need to free it to not leak memory.

	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:

	    15536
	    15760
	    15920
	    16144
	    16304
	    16480
	    16640
	    16784
	    17008
	    17280

	After:

	    15584
	    15584
	    15760
	    15824
	    15888
	    15888
	    15888
	    15888
	    16048
	    16112
	---
	 regparse.c | 3 ++-
	 1 file changed, 2 insertions(+), 1 deletion(-)