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Change qb cap to prevent side-fold collapses.

Previously, in a stereo split with itheta==16384, but without
 enough bits left over to actually code a pulse, the target band
 would completely collapse, because the mid gain would be zero and
 we don't fold the side.
This changes the limit to ensure that we never set qn>1 unless we
 know we'll have enough bits for at least one pulse.
This should eliminate the last possible whole-band collapse.

--- a/libcelt/bands.c

+++ b/libcelt/bands.c

@@ -594,7 +594,7 @@

       }

 }

-static int compute_qn(int N, int b, int offset, int stereo)

+static int compute_qn(int N, int b, int offset, int pulse_cap, int stereo)

 {

    static const celt_int16 exp2_table8[8] =

       {16384, 17866, 19483, 21247, 23170, 25267, 27554, 30048};

@@ -602,9 +602,12 @@

    int N2 = 2*N-1;

    if (stereo && N==2)

       N2--;

-   qb = IMIN((b>>1)-(1<<BITRES), (b+N2*offset)/N2);

+   /* The upper limit ensures that in a stereo split with itheta==16384, we'll

+       always have enough bits left over to code at least one pulse in the

+       side; otherwise it would collapse, since it doesn't get folded. */

+   qb = IMIN(b-pulse_cap-(4<<BITRES), (b+N2*offset)/N2);

-   qb = IMAX(0, IMIN(8<<BITRES, qb));

+   qb = IMIN(8<<BITRES, qb);

    if (qb<(1<<BITRES>>1)) {

       qn = 1;

@@ -748,13 +751,15 @@

       int itheta=0;

       int mbits, sbits, delta;

       int qalloc;

+      int pulse_cap;

       int offset;

       int orig_fill;

       celt_int32 tell;

       /* Decide on the resolution to give to the split parameter theta */

-      offset = ((m->logN[i]+(LM<<BITRES))>>1) - (stereo ? QTHETA_OFFSET_STEREO : QTHETA_OFFSET);

-      qn = compute_qn(N, b, offset, stereo);

+      pulse_cap = m->logN[i]+(LM<<BITRES);

+      offset = (pulse_cap>>1) - (stereo ? QTHETA_OFFSET_STEREO : QTHETA_OFFSET);

+      qn = compute_qn(N, b, offset, pulse_cap, stereo);

       if (stereo && i>=intensity)

          qn = 1;

       if (encode)