object.c: raise on long invalid float string

	* object.c (rb_cstr_to_dbl_raise): check long invalid float
	  string more precisely when truncating insignificant part.
	  [ruby-core:86800] [Bug #14729]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@66880 b2dd03c8-39d4-4d8f-98ff-823fe69b080e