obs-outputs: Fix FLV corruption bug

The 'sent_headers' member variable of the FLV output would not be reset
when the output was restarted, causing important data to not be written,
thus creating an invalid FLV file.

plugins/obs-outputs/flv-output.c

@@ -79,6 +79,7 @@ static void flv_output_stop(void *data)
 		fclose(stream->file);
 		obs_output_end_data_capture(stream->output);
 		stream->active = false;
+		stream->sent_headers = false;
 
 		info("FLV file output complete");
 	}