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

Work around -Werror=catch-value.

NEWS

@@ -2,6 +2,7 @@ ver 0.20.12 (not yet released)
 * input
   - curl: fix seeking
 * decoder
+  - ffmpeg: fix GCC 8 warning
   - vorbis: fix Tremor support
 * player
   - log message when decoder is too slow

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;

test/test_protocol.cxx

@@ -37,7 +37,7 @@ ArgParserTest::TestRange()
 	try {
 		range = ParseCommandArgRange("-2");
 		CPPUNIT_ASSERT(false);
-	} catch (ProtocolError) {
+	} catch (const ProtocolError &) {
 		CPPUNIT_ASSERT(true);
 	}
 }