decoder/ffmpeg, test/test_protocol: catch exceptions by reference

Work around -Werror=catch-value.

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -258,7 +258,7 @@ FfmpegSendFrame(DecoderClient &client, InputStream &is,
 	try {
 		output_buffer = copy_interleave_frame(codec_context, frame,
 						      buffer);
-	} catch (const std::exception e) {
+	} catch (const std::exception &e) {
 		/* this must be a serious error, e.g. OOM */
 		LogError(e);
 		return DecoderCommand::STOP;