Qt: fix a potential memory leak in RTP audio stream playback

Change-Id: I6847f85d56841f99594063bfb7441d0c40e452a7
Reviewed-on: https://code.wireshark.org/review/12425
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>

ui/qt/rtp_audio_stream.cpp

@@ -227,7 +227,7 @@ void RtpAudioStream::decode()
         if (decoded_bytes == 0 || sample_rate == 0) {
             // We didn't decode anything. Clean up and prep for the next packet.
             last_sequence = rtp_packet->info->info_seq_num;
-
+            g_free(decode_buff);
             continue;
         }