[Bug #18154] Fix memory leak in String#initialize

	String#initialize can leak memory when called on a string that is marked
	with STR_NOFREE because it does not unset the STR_NOFREE flag.
	---
	 string.c                 |  2 +-
	 test/ruby/test_string.rb | 10 ++++++++++
	 2 files changed, 11 insertions(+), 1 deletion(-)