date_core.c: defensive code

	* ext/date/date_core.c (f_cmp): check comparison failure.

	* ext/date/date_core.c (d_lite_step): deal with the comparison
	  result more defensively.  [ruby-core:85796] [Bug #14549]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@62883 b2dd03c8-39d4-4d8f-98ff-823fe69b080e