apply the proposed fix for bug 8602

http://bugs.digium.com/view.php?id=8602
(i am not sure if there is still missing cast in
front of the alloca() call - being a macro this is
probably triggered only when actually used).

Add function ast_str_reset() to reinitialize the
string to an empty string instead of playing with
the internal fields.



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48560 65c4cc65-6c06-0410-ace0-fbb531ad65f3