strscan.c: fix segfault in aref

	* ext/strscan/strscan.c (strscan_aref): fix segfault after
	  get_byte or getch which do not apply regexp.
	  [ruby-core:82116] [Bug #13759]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_3@59545 b2dd03c8-39d4-4d8f-98ff-823fe69b080e