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Fixes an overflow in limit_warped_coefs()

For large values of maxabs_Q20, silk_MUL( maxabs_Q20, ind + 1 ) could
overflow.

--- a/silk/fixed/noise_shape_analysis_FIX.c

+++ b/silk/fixed/noise_shape_analysis_FIX.c

@@ -65,6 +65,7 @@

     opus_int   i, iter, ind = 0;

     opus_int32 tmp, maxabs_Q24, chirp_Q16, gain_Q16;

     opus_int32 nom_Q16, den_Q24;

+    opus_int32 limit_Q20, maxabs_Q20;

     /* Convert to monic coefficients */

     lambda_Q16 = -lambda_Q16;

@@ -78,7 +79,7 @@

     for( i = 0; i < order; i++ ) {

         coefs_Q24[ i ] = silk_SMULWW( gain_Q16, coefs_Q24[ i ] );

     }

-

+    limit_Q20 = silk_RSHIFT(limit_Q24, 4);

     for( iter = 0; iter < 10; iter++ ) {

         /* Find maximum absolute value */

         maxabs_Q24 = -1;

@@ -89,7 +90,9 @@

                 ind = i;

             }

         }

-        if( maxabs_Q24 <= limit_Q24 ) {

+        /* Use Q20 to avoid any overflow when multiplying by (ind + 1) later. */

+        maxabs_Q20 = silk_RSHIFT(maxabs_Q24, 4);

+        if( maxabs_Q20 <= limit_Q20 ) {

             /* Coefficients are within range - done */

             return;

         }

@@ -105,8 +108,8 @@

         /* Apply bandwidth expansion */

         chirp_Q16 = SILK_FIX_CONST( 0.99, 16 ) - silk_DIV32_varQ(

-            silk_SMULWB( maxabs_Q24 - limit_Q24, silk_SMLABB( SILK_FIX_CONST( 0.8, 10 ), SILK_FIX_CONST( 0.1, 10 ), iter ) ),

-            silk_MUL( maxabs_Q24, ind + 1 ), 22 );

+            silk_SMULWB( maxabs_Q20 - limit_Q20, silk_SMLABB( SILK_FIX_CONST( 0.8, 10 ), SILK_FIX_CONST( 0.1, 10 ), iter ) ),

+            silk_MUL( maxabs_Q20, ind + 1 ), 22 );

         silk_bwexpander_32( coefs_Q24, order, chirp_Q16 );

         /* Convert to monic warped coefficients */