chan_sip: Fix lastrtprx always updated

Packets are read regulary, when there is no data in buffer fr->frametype
is AST_FRAME_NULL. There was no check of frametype and lastrtprx always 
updated and, therefore, rtptimeout did not work at all.

ASTERISK-25270 #close

Change-Id: If3b5ca0dbb822582a86eb7d01dcae4e83448c41d

channels/chan_sip.c

@@ -8598,7 +8598,9 @@ static struct ast_frame *sip_read(struct ast_channel *ast)
 
 	sip_pvt_lock(p);
 	fr = sip_rtp_read(ast, p, &faxdetected);
-	p->lastrtprx = time(NULL);
+	if (fr && fr->frametype != AST_FRAME_NULL) {
+		p->lastrtprx = time(NULL);
+	}
 
 	/* If we detect a CNG tone and fax detection is enabled then send us off to the fax extension */
 	if (faxdetected && ast_test_flag(&p->flags[1], SIP_PAGE2_FAX_DETECT_CNG)) {