SDP: set audio ch count 2 for Opus

According to RFC 7587, channel count must be set always to 2 (actual
channel count like mono is signalized in-band in Opus)

src/rtsp/BasicRTSPOnlySubsession.cpp

@@ -149,6 +149,8 @@ void BasicRTSPOnlySubsession::setSDPLines() {
 		char* rtpmapLine = strdup("a=rtpmap:97 PCMU/48000/2\n"); //only to alloc max possible size
 		//char const* auxSDPLine = "";
 
+		const int sdp_ch_count = audio_codec == AC_OPUS ? 2 :
+			audio_channels; // RFC 7587 enforces 2 for Opus
 		char const* const sdpFmt = "m=%s %u RTP/AVP %u\r\n"
 				"c=IN IP4 %s\r\n"
 				"b=AS:%u\r\n"
@@ -172,7 +174,7 @@ void BasicRTSPOnlySubsession::setSDPLines() {
 				rtpPayloadType,
 				audio_codec == AC_MULAW ? "PCMU" : audio_codec == AC_ALAW ? "PCMA" : "opus",
 				audio_sample_rate,
-				audio_channels,
+				sdp_ch_count,
 				trackId()); // a=control:<track-id>
 
 		fSDPLines = sdpLines;

src/utils/sdp.c

@@ -251,7 +251,11 @@ int sdp_add_audio(bool ipv6, int port, int sample_rate, int channels, audio_code
                 return -2;
 	}
 
-	snprintf(sdp_state->stream[index].rtpmap, STR_LENGTH, "a=rtpmap:%d %s/%i/%i\n", PT_DynRTP_Type97, audio_codec, ts_rate, channels);
+        const int sdp_ch_count =
+            codec == AC_OPUS ? 2 : channels; // RFC 7587 enforces 2 for Opus
+        snprintf(sdp_state->stream[index].rtpmap, STR_LENGTH,
+                 "a=rtpmap:%d %s/%i/%i\n", PT_DynRTP_Type97, audio_codec,
+                 ts_rate, sdp_ch_count);
     }
     sdp_state->audio_set = true;
     start();