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Fix a CELT encoder CBR corner case

If configuring CELT for CBR but controlling the bitrate with
OPUS_SET_BITRATE rather than nbCompressedBytes, then the range
coder buffer would never get resized. AFAICT this could never
be triggered in Opus because CBR was also controlled by
nbCompressedBytes.

--- a/celt/celt_encoder.c

+++ b/celt/celt_encoder.c

@@ -1567,8 +1567,11 @@

       if (tell>1)

          tmp += tell*mode->Fs;

       if (st->bitrate!=OPUS_BITRATE_MAX)

+      {

          nbCompressedBytes = IMAX(2, IMIN(nbCompressedBytes,

                (tmp+4*mode->Fs)/(8*mode->Fs)-!!st->signalling));

+         ec_enc_shrink(enc, nbCompressedBytes);

+      }

       effectiveBytes = nbCompressedBytes - nbFilledBytes;

    }

    equiv_rate = ((opus_int32)nbCompressedBytes*8*50 << (3-LM)) - (40*C+20)*((400>>LM) - 50);

--- a/src/opus_encoder.c

+++ b/src/opus_encoder.c

@@ -2129,7 +2129,7 @@

            if (redundancy && celt_to_silk && st->mode==MODE_HYBRID && st->use_vbr)

            {

               OPUS_MOVE(data+ret, data+nb_compr_bytes, redundancy_bytes);

-              nb_compr_bytes = nb_compr_bytes+redundancy_bytes;

+              nb_compr_bytes = ret+redundancy_bytes;

            }

         }

     }

-- 

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