array.c: finish_exact_sum

	* array.c (finish_exact_sum): extract duplicate code from
	  rb_ary_sum.

	array.c: check if numeric

	* array.c (finish_exact_sum): add 0 and the initial value to check
	  if the latter is numeric.  [ruby-core:79572] [Bug #13222]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@61361 b2dd03c8-39d4-4d8f-98ff-823fe69b080e