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Fixes a corner case that was causing silk_A2NLSF() to fail

Input that caused the problem was:
a_Q16[] = [129763 -49346 -42032 5488 28014 -1549 -2384 -8568
3154 -5104 -1605 6291 11861 -1843 -17715 7361]

--- a/silk/A2NLSF.c

+++ b/silk/A2NLSF.c

@@ -129,7 +129,7 @@

 {

     opus_int      i, k, m, dd, root_ix, ffrac;

     opus_int32 xlo, xhi, xmid;

-    opus_int32 ylo, yhi, ymid;

+    opus_int32 ylo, yhi, ymid, thr;

     opus_int32 nom, den;

     opus_int32 P[ SILK_MAX_ORDER_LPC / 2 + 1 ];

     opus_int32 Q[ SILK_MAX_ORDER_LPC / 2 + 1 ];

@@ -161,6 +161,7 @@

     }

     k = 1;                          /* Loop counter */

     i = 0;                          /* Counter for bandwidth expansions applied */

+    thr = 0;

     while( 1 ) {

         /* Evaluate polynomial */

 #if OVERSAMPLE_COSINE_TABLE

@@ -173,7 +174,14 @@

         yhi = silk_A2NLSF_eval_poly( p, xhi, dd );

         /* Detect zero crossing */

-        if( ( ylo <= 0 && yhi >= 0 ) || ( ylo >= 0 && yhi <= 0 ) ) {

+        if( ( ylo <= 0 && yhi >= thr ) || ( ylo >= 0 && yhi <= -thr ) ) {

+            if( yhi == 0 ) {

+                /* If the root lies exactly at the end of the current       */

+                /* interval, look for the next root in the next interval    */

+                thr = 1;

+            } else {

+                thr = 0;

+            }

             /* Binary division */

 #if OVERSAMPLE_COSINE_TABLE

             ffrac = -128;

@@ -220,8 +228,7 @@

             NLSF[ root_ix ] = (opus_int16)silk_min_32( silk_LSHIFT( (opus_int32)k, 8 ) + ffrac, silk_int16_MAX );

 #endif

-            silk_assert( NLSF[ root_ix ] >=     0 );

-            silk_assert( NLSF[ root_ix ] <= 32767 );

+            silk_assert( NLSF[ root_ix ] >= 0 );

             root_ix++;        /* Next root */

             if( root_ix >= d ) {

@@ -243,8 +250,9 @@

         } else {

             /* Increment loop counter */

             k++;

-            xlo    = xhi;

-            ylo    = yhi;

+            xlo = xhi;

+            ylo = yhi;

+            thr = 0;

 #if OVERSAMPLE_COSINE_TABLE

             if( k > 2 * LSF_COS_TAB_SZ_FIX ) {