bignum.c (estimate_initial_sqrt): prevent integer overflow

	`Integer.sqrt(0xffff_ffff_ffff_ffff ** 2)` caused assertion failure
	because of integer overflow.  [ruby-core:95453] [Bug #16269]
	---
	 bignum.c                  | 10 +++++++++-
	 test/ruby/test_integer.rb |  3 +++
	 2 files changed, 12 insertions(+), 1 deletion(-)

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67894 b2dd03c8-39d4-4d8f-98ff-823fe69b080e