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.
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Merged revisions 374758 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 374763 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 374771 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@374778 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/pbx.c

@@ -6026,6 +6026,9 @@ static enum ast_pbx_result __ast_pbx_run(struct ast_channel *c,
 		int invalid = 0;
 		int timeout = 0;
 
+		/* No digits pressed yet */
+		dst_exten[pos] = '\0';
+
 		/* loop on priorities in this context/exten */
 		while (!(res = ast_spawn_extension(c, ast_channel_context(c), ast_channel_exten(c), ast_channel_priority(c),
 			S_COR(ast_channel_caller(c)->id.number.valid, ast_channel_caller(c)->id.number.str, NULL),