Fix execution of 'i' extension due to uninitialized variable.

Fix execution of 'i' extension due to uninitialized variable.

The fix for ASTERISK-18243 added code that could potentially use
dst_exten[] uninitialized.  As a result the 'i' exten may not be executed
when it should.

(closes issue ASTERISK-20455)
Reported by: Richard Miller
Patches:
      pbx-1.8.16.0.diff (license #5685) patch uploaded by Richard Miller
      Made some cosmetic modifications.
........

Merged revisions 374758 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@374763 65c4cc65-6c06-0410-ace0-fbb531ad65f3