don't try to get the length of the string in the ast_dynamic_str object unless...

don't try to get the length of the string in the ast_dynamic_str object unless we know there is actually one there


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@42185 65c4cc65-6c06-0410-ace0-fbb531ad65f3