rtpspeexpay: Do not transmitt samples with GAP flag

If we get GAP samples, there is no need to transmitt it.
In some situations, microphone is muted, we can drop net traffick
usage to ~1 kbit/s. Without patch it will stay ~20 kbit/s

gst/rtp/gstrtpspeexpay.c

@@ -260,6 +260,11 @@ gst_rtp_speex_pay_handle_buffer (GstBaseRTPPayload * basepayload,
       break;
   }
 
+  if (GST_BUFFER_FLAG_IS_SET (buffer, GST_BUFFER_FLAG_GAP)) {
+    ret = GST_FLOW_OK;
+    goto done;
+  }
+
   timestamp = GST_BUFFER_TIMESTAMP (buffer);
   duration = GST_BUFFER_DURATION (buffer);