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Don't try to create fullband silk frames when forced to low rate.

When libopus is forced to sufficiently low rates it will start
outputting 'PLC' (one byte) frames. The code that did this
did not sanitize the mode well enough and would create corrupted
TOC values in some cases.

--- a/src/opus_encoder.c

+++ b/src/opus_encoder.c

@@ -504,7 +504,9 @@

     if (max_data_bytes<3 || st->bitrate_bps < 3*frame_rate*8

        || (frame_rate<50 && (max_data_bytes*frame_rate<300 || st->bitrate_bps < 2400)))

     {

+       /*If the space is too low to do something useful, emit 'PLC' frames.*/

        int tocmode = st->mode;

+       int bw = st->bandwidth == 0 ? OPUS_BANDWIDTH_NARROWBAND : st->bandwidth;

        if (tocmode==0)

           tocmode = MODE_SILK_ONLY;

        if (frame_rate>100)

@@ -511,9 +513,13 @@

           tocmode = MODE_CELT_ONLY;

        if (frame_rate < 50)

           tocmode = MODE_SILK_ONLY;

-       data[0] = gen_toc(tocmode, frame_rate,

-                         st->bandwidth == 0 ? OPUS_BANDWIDTH_NARROWBAND : st->bandwidth,

-                         st->stream_channels);

+       if(tocmode==MODE_SILK_ONLY&&bw>OPUS_BANDWIDTH_WIDEBAND)

+          bw=OPUS_BANDWIDTH_WIDEBAND;

+       else if (tocmode==MODE_CELT_ONLY&&bw==OPUS_BANDWIDTH_MEDIUMBAND)

+          bw=OPUS_BANDWIDTH_NARROWBAND;

+       else if (bw<=OPUS_BANDWIDTH_SUPERWIDEBAND)

+          bw=OPUS_BANDWIDTH_SUPERWIDEBAND;

+       data[0] = gen_toc(tocmode, frame_rate, bw, st->stream_channels);

        RESTORE_STACK;

        return 1;

     }