mgcp: fix "L: a:" header parsing: heed ";" separator

In the "L: a:" header, read the first codec name only up to the ";"
separator, and ignore the rest.

According to RFC-2705, the "L: a:" header may include multiple codecs
like "GSM-EFR;GSM" in:

  L: p:20, a:GSM-EFR;GSM, nt:IN

osmo-mgw can handle only a single codec here. Since recently, osmo-msc
is our first client that may actually send more than one codec. This
uncovered a bug that leads to failing voice calls:

* osmo-mgw parses the entire list "GSM-EFR;GSM" as a single codec name,
* puts that into the ptmap without scrutiny,
* and even sends it back in the OK response, in the *SDP* part, as a
  single "GSM-EFR;GSM" codec entry.

We do not care very much about the "a:" codec list, because we always
establish codecs via SDP later. So all we need to fix this is: parse the
first codec done correctly, and ignore the rest.

Related: OS#6081
Change-Id: I0342e85b32ed89f3a1fdb6131c3c8ded8f47a455

src/libosmo-mgcp/mgcp_protocol.c

@@ -672,8 +672,8 @@ static int set_local_cx_options(void *ctx, struct mgcp_lco *lco,
 		case 'a':
 			/* FIXME: LCO also supports the negotiation of more than one codec.
 			 * (e.g. a:PCMU;G726-32) But this implementation only supports a single
-			 * codec only. */
-			if (sscanf(lco_id + 1, ":%16[^,]", codec) == 1) {
+			 * codec only. Ignoring all but the first codec. */
+			if (sscanf(lco_id + 1, ":%16[^,;]", codec) == 1) {
 				talloc_free(lco->codec);
 				/* MGCP header is case insensive, and we'll need
 				   codec in uppercase when using it later: */