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Fixes fixed-point PLC issue reported in trac ticket #1954

A fixed shift factor was insufficient to properly estimate the decay
factor, resulting in extreme attenuation of the PLC excitation.

--- a/celt/celt_decoder.c

+++ b/celt/celt_decoder.c

@@ -506,14 +506,17 @@

          {

             opus_val32 E1=1, E2=1;

             int decay_length;

+#ifdef FIXED_POINT

+            int shift = IMAX(0,2*celt_zlog2(celt_maxabs16(&exc[MAX_PERIOD-exc_length], exc_length))-20);

+#endif

             decay_length = exc_length>>1;

             for (i=0;i<decay_length;i++)

             {

                opus_val16 e;

                e = exc[MAX_PERIOD-decay_length+i];

-               E1 += SHR32(MULT16_16(e, e), 8);

+               E1 += SHR32(MULT16_16(e, e), shift);

                e = exc[MAX_PERIOD-2*decay_length+i];

-               E2 += SHR32(MULT16_16(e, e), 8);

+               E2 += SHR32(MULT16_16(e, e), shift);

             }

             E1 = MIN32(E1, E2);

             decay = celt_sqrt(frac_div32(SHR32(E1, 1), E2));