Fix chan_iax2 to not report an RDNIS number if it is blank.

Some ISDN switches complain or block the call if the RDNIS number is
empty.

* Made chan_iax2 not save a RDNIS number into the ast_channel if the
string is blank.  This is what other channel drivers do.

(closes issue ASTERISK-17152)
Reported by: rmudgett
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Merged revisions 348735 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 348736 from http://svn.asterisk.org/svn/asterisk/branches/10


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@348737 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_iax2.c

@@ -5835,8 +5835,10 @@ static struct ast_channel *ast_iax2_new(int callno, int state, iax2_format capab
 		tmp->caller.ani.number.str = ast_strdup(i->cid_num);
 	}
 	tmp->dialed.number.str = ast_strdup(i->dnid);
-	tmp->redirecting.from.number.valid = 1;
-	tmp->redirecting.from.number.str = ast_strdup(i->rdnis);
+	if (!ast_strlen_zero(i->rdnis)) {
+		tmp->redirecting.from.number.valid = 1;
+		tmp->redirecting.from.number.str = ast_strdup(i->rdnis);
+	}
 	tmp->caller.id.name.presentation = i->calling_pres;
 	tmp->caller.id.number.presentation = i->calling_pres;
 	tmp->caller.id.number.plan = i->calling_ton;