decoder/ffmpeg: fix crash on out-of-memory

src/decoder/plugins/FfmpegDecoderPlugin.cxx

@@ -340,15 +340,19 @@ ffmpeg_send_packet(Decoder &decoder, InputStream &is,
 			break;
 		}
 
-		uint8_t *output_buffer;
+		uint8_t *output_buffer = nullptr;
 		int audio_size = 0;
 		if (got_frame) {
 			audio_size = copy_interleave_frame(codec_context,
 							   frame,
 							   &output_buffer,
 							   buffer, buffer_size);
-			if (audio_size < 0)
-				len = audio_size;
+			if (audio_size < 0) {
+				/* this must be a serious error,
+				   e.g. OOM */
+				LogFfmpegError(audio_size);
+				return DecoderCommand::STOP;
+			}
 		}
 
 		packet2.data += len;