Replace PATCHWELCOME by relevant error codes.

This supersedes commit 154c0a8c.
The bug was also reported by Rafaël Carré.

libavcodec/libcelt_dec.c

@@ -40,7 +40,7 @@ static int ff_celt_error_to_averror(int err)
 #endif
         case CELT_INTERNAL_ERROR:   return AVERROR(EFAULT);
         case CELT_CORRUPTED_DATA:   return AVERROR_INVALIDDATA;
-        case CELT_UNIMPLEMENTED:    return AVERROR_PATCHWELCOME;
+        case CELT_UNIMPLEMENTED:    return AVERROR(ENOSYS);
 #ifdef ENOTRECOVERABLE
         case CELT_INVALID_STATE:    return AVERROR(ENOTRECOVERABLE);
 #endif

libavformat/sbgdec.c

@@ -1467,7 +1467,7 @@ static int sbg_read_seek2(AVFormatContext *avf, int stream_index,
                           int64_t min_ts, int64_t ts, int64_t max_ts, int flags)
 {
     if (flags || stream_index > 0)
-        return AVERROR_PATCHWELCOME;
+        return AVERROR(EINVAL);
     if (stream_index < 0)
         ts = av_rescale_q(ts, AV_TIME_BASE_Q, avf->streams[0]->time_base);
     avf->streams[0]->cur_dts = ts;