enable don't select audio device name when start agora audio share

@@ -1257,10 +1257,16 @@
 				cmd.type = 63;
 				var micSel = document.getElementById("aac_mics");
 				var index = micSel.selectedIndex;
-				var sel_mic_name = micSel.options[index].text;
+				if (index >= 0) {
+					var sel_mic_name = micSel.options[index].text;
+
+					cmd.data = utf16to8(sel_mic_name);
+					cmd.data_len = cmd.data.length;
+				}
+				else {
+					cmd.data_len = 0;
+				}
 
-				cmd.data = utf16to8(sel_mic_name);
-				cmd.data_len = cmd.data.length;
 			}
 			else if(cmdName == "stop_audio_share"){
 				cmd.type = 64;