Fix SDP negotiation for audio with asymmetric encoding parameters (#4622)

To fix #4615.

Currently the library assumes to always send offer without encoding param when channel count is 1. But alas, app may modify the SDP or 3rd party media may creates its own SDP.

Author: nanangizz

pjmedia/src/pjmedia/sdp_neg.c

@@ -778,6 +778,7 @@ static pj_status_t process_m_answer( pj_pool_t *pool,
                                      pj_bool_t allow_asym)
 {
     unsigned i;
+    pj_bool_t is_audio;
 
     /* Check that the media type match our offer. */
 
@@ -786,6 +787,8 @@ static pj_status_t process_m_answer( pj_pool_t *pool,
         return PJMEDIA_SDPNEG_EINVANSMEDIA;
     }
 
+    is_audio = (pj_strcmp2(&offer->desc.media, "audio") == 0);
+
     /* Check if remote has rejected our offer */
     if (answer->desc.port == 0) {
 
@@ -886,6 +889,10 @@ static pj_status_t process_m_answer( pj_pool_t *pool,
                 }
                 pjmedia_sdp_attr_get_rtpmap(a, &or_);
 
+                /* For audio, channel count is 1 if not specified */
+                if (is_audio && or_.param.slen == 0)
+                    or_.param = pj_str("1");
+
                 /* Find paylaod in answer SDP with matching 
                  * encoding name and clock rate.
                  */
@@ -896,13 +903,16 @@ static pj_status_t process_m_answer( pj_pool_t *pool,
                         pjmedia_sdp_rtpmap ar;
                         pjmedia_sdp_attr_get_rtpmap(a, &ar);
 
+                        /* For audio, channel count is 1 if not specified */
+                        if (is_audio && ar.param.slen == 0)
+                            ar.param = pj_str("1");
+
                         /* See if encoding name, clock rate, and channel
                          * count match 
                          */
                         if (!pj_stricmp(&or_.enc_name, &ar.enc_name) &&
                             or_.clock_rate == ar.clock_rate &&
-                            (pj_stricmp(&or_.param, &ar.param)==0 ||
-                             (ar.param.slen==1 && *ar.param.ptr=='1')))
+                            (pj_stricmp(&or_.param, &ar.param)==0))
                         {
                             /* Call custom format matching callbacks */
                             if (custom_fmt_match(pool, &or_.enc_name,