mgcp: simplify getting msgb tail in mgcp_msg_terminate_nul()

The current statement:

  msg->l2h + msgb_l2len(msg)

looks as follows, if we expand the msgb_l2len():

  msg->l2h + msgb->tail - msg->l2h

so this is basically equal to msgb->tail alone.

Change-Id: I4f4b0f792bbeef94a5449c4a5843628a703a3d54
Related: CID#272990

include/osmocom/mgcp/mgcp_common.h

@@ -68,7 +68,7 @@ struct mgcp_codec_param {
 /* Ensure that the msg->l2h is NUL terminated. */
 static inline int mgcp_msg_terminate_nul(struct msgb *msg)
 {
-	unsigned char *tail = msg->l2h + msgb_l2len(msg); /* char after l2 data */
+	unsigned char *tail = msg->tail; /* char after l2 data */
 	if (tail[-1] == '\0')
 		/* nothing to do */;
 	else if (msgb_tailroom(msg) > 0)