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Making the PLC robust to overflows caused by extreme bit-streams

This is not strictly necessary because the only streams that cause these
already sound horrible, but it helps the testing process.

--- a/silk/PLC.c

+++ b/silk/PLC.c

@@ -183,7 +183,11 @@

     opus_int16 sLTP[ MAX_FRAME_LENGTH ];

     opus_int32 sLTP_Q14[ 2 * MAX_FRAME_LENGTH ];

     silk_PLC_struct *psPLC = &psDec->sPLC;

+    opus_int32 prevGain_Q10[2];

+    prevGain_Q10[0] = silk_RSHIFT( psPLC->prevGain_Q16[ 0 ], 6);

+    prevGain_Q10[1] = silk_RSHIFT( psPLC->prevGain_Q16[ 1 ], 6);

+

     if( psDec->first_frame_after_reset ) {

        silk_memset( psPLC->prevLPC_Q12, 0, sizeof( psPLC->prevLPC_Q12 ) );

     }

@@ -193,8 +197,8 @@

     exc_buf_ptr = exc_buf;

     for( k = 0; k < 2; k++ ) {

         for( i = 0; i < psPLC->subfr_length; i++ ) {

-            exc_buf_ptr[ i ] = (opus_int16)silk_RSHIFT(

-                silk_SMULWW( psDec->exc_Q14[ i + ( k + psPLC->nb_subfr - 2 ) * psPLC->subfr_length ], psPLC->prevGain_Q16[ k ] ), 14 );

+            exc_buf_ptr[ i ] = (opus_int16)silk_SAT16( silk_RSHIFT(

+                silk_SMULWW( psDec->exc_Q14[ i + ( k + psPLC->nb_subfr - 2 ) * psPLC->subfr_length ], prevGain_Q10[ k ] ), 8 ) );

         }

         exc_buf_ptr += psPLC->subfr_length;

     }

@@ -336,7 +340,7 @@

         sLPC_Q14_ptr[ MAX_LPC_ORDER + i ] = silk_ADD_LSHIFT32( sLPC_Q14_ptr[ MAX_LPC_ORDER + i ], LPC_pred_Q10, 4 );

         /* Scale with Gain */

-        frame[ i ] = (opus_int16)silk_SAT16( silk_RSHIFT_ROUND( silk_SMULWW( sLPC_Q14_ptr[ MAX_LPC_ORDER + i ], psPLC->prevGain_Q16[ 1 ] ), 14 ) );

+        frame[ i ] = (opus_int16)silk_SAT16( silk_SAT16( silk_RSHIFT_ROUND( silk_SMULWW( sLPC_Q14_ptr[ MAX_LPC_ORDER + i ], prevGain_Q10[ 1 ] ), 8 ) ) );

     }

     /* Save LPC state */