merge revision(s) 1b1ea7b3,b6e68079: [Backport #17092]

	Fix Array#flatten for recursive array when given positive depth [Bug
	 #17092]

	---
	 array.c                 | 44 ++++++++++++++++++++++++++------------------
	 test/ruby/test_array.rb | 14 +++++++++++---
	 2 files changed, 37 insertions(+), 21 deletions(-)

	Initialize memo pointer and use it consistently to silence gcc 7+

	---
	 array.c | 10 +++++-----
	 1 file changed, 5 insertions(+), 5 deletions(-)