Fix for rpid (from airport, bug #2910)

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@4323 65c4cc65-6c06-0410-ace0-fbb531ad65f3

channels/chan_sip.c

@@ -5259,7 +5259,8 @@ static int get_rpid_num(char *input,char *output, int maxlen)
 	end = strchr(output,'@');
 	if (end)
 		*end = '\0';
-
+	else
+		output[0] = '\0';
 	if(strstr(input,"privacy=full") || strstr(input,"privacy=uri"))
 		return AST_PRES_PROHIB_USER_NUMBER_NOT_SCREENED;