mgcp: Initialize the tone to CHAR_MAX as this might not be a request

The RQNT message might not contain a 'S:' line with the actual tone
to play. Instead of calling the callback with the 0 as tone just leave
early.

Example:
X: 6B9519B88F0
R: D/[0-9#*](N), G/ft, fxr/t38

openbsc/src/libmgcp/mgcp_protocol.c

@@ -862,7 +862,7 @@ static struct msgb *handle_noti_req(struct mgcp_parse_data *p)
 {
 	int res = 0;
 	char *line;
-	char tone = 0;
+	char tone = CHAR_MAX;
 
 	if (p->found != 0)
 		return create_err_response(NULL, 400, "RQNT", p->trans);