Reset our ast_str before passing it on to dialplan function backends.

Reset our ast_str before passing it on to dialplan function backends.

It is possible for a dialplan backend to not modify the given buffer or ast_str
and still return success. This causes any previous value stored in the buffer
to be used as if the new function call provided it. Some functions also append
to the given buffer assuming it is empty.

The test_substitution unit test has also been modified to detect this problem.

(closes issue ASTERISK-17878)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@327106 65c4cc65-6c06-0410-ace0-fbb531ad65f3