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Fix several memory errors in the SILK resampler.

1) The memcpy's were using sizeof(opus_int32), but the type of the
    local buffer was opus_int16.
2) Because the size was wrong, this potentially allowed the source
    and destination regions of the memcpy overlap.
   I _believe_ that nSamplesIn is at least fs_in_khZ, which is at
    least 8.
   Since RESAMPLER_ORDER_FIR_12 is only 8, I don't think that's a
    problem once you fix the type size.
3) The size of the buffer used RESAMPLER_MAX_BATCH_SIZE_IN, but the
    data stored in it was actually _twice_ the input batch size
    (nSamplesIn<<1).

Because this never blew up in testing, I suspect that in practice
 the batch sizes are reasonable enough that none of these things
 was ever a problem, but proving that seems non-obvious.

This patch just converts the whole thing to use CELT's vararrays.
This fixes the buffer size problems (since we allocate a buffer
 with the actual size we use) and gets these large buffers off the
 stack on devices using the pseudo-stack.
It also fixes the memcpy problems by changing the sizeof to
 opus_int16.
It turns out sFIR, which saved state between calls, was being used
 elsewhere as opus_int32, so this converts it to a union to make
 this sharing explicit.

--- a/silk/resampler_private_IIR_FIR.c

+++ b/silk/resampler_private_IIR_FIR.c

@@ -31,6 +31,7 @@

 #include "SigProc_FIX.h"

 #include "resampler_private.h"

+#include "stack_alloc.h"

 static inline opus_int16 *silk_resampler_private_IIR_FIR_INTERPOL(

     opus_int16  *out,

@@ -71,10 +72,13 @@

     silk_resampler_state_struct *S = (silk_resampler_state_struct *)SS;

     opus_int32 nSamplesIn;

     opus_int32 max_index_Q16, index_increment_Q16;

-    opus_int16 buf[ RESAMPLER_MAX_BATCH_SIZE_IN + RESAMPLER_ORDER_FIR_12 ];

+    VARDECL( opus_int16, buf );

+    SAVE_STACK;

+    ALLOC( buf, 2 * S->batchSize + RESAMPLER_ORDER_FIR_12, opus_int16 );

+

     /* Copy buffered samples to start of buffer */

-    silk_memcpy( buf, S->sFIR, RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );

+    silk_memcpy( buf, S->sFIR.i16, RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );

     /* Iterate over blocks of frameSizeIn input samples */

     index_increment_Q16 = S->invRatio_Q16;

@@ -91,7 +95,7 @@

         if( inLen > 0 ) {

             /* More iterations to do; copy last part of filtered signal to beginning of buffer */

-            silk_memcpy( buf, &buf[ nSamplesIn << 1 ], RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );

+            silk_memcpy( buf, &buf[ nSamplesIn << 1 ], RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );

         } else {

             break;

         }

@@ -98,6 +102,6 @@

     }

     /* Copy last part of filtered signal to the state for the next call */

-    silk_memcpy( S->sFIR, &buf[ nSamplesIn << 1 ], RESAMPLER_ORDER_FIR_12 * sizeof( opus_int32 ) );

+    silk_memcpy( S->sFIR.i16, &buf[ nSamplesIn << 1 ], RESAMPLER_ORDER_FIR_12 * sizeof( opus_int16 ) );

+    RESTORE_STACK;

 }

-

--- a/silk/resampler_private_down_FIR.c

+++ b/silk/resampler_private_down_FIR.c

@@ -155,7 +155,7 @@

     const opus_int16 *FIR_Coefs;

     /* Copy buffered samples to start of buffer */

-    silk_memcpy( buf, S->sFIR, S->FIR_Order * sizeof( opus_int32 ) );

+    silk_memcpy( buf, S->sFIR.i32, S->FIR_Order * sizeof( opus_int32 ) );

     FIR_Coefs = &S->Coefs[ 2 ];

@@ -185,5 +185,5 @@

     }

     /* Copy last part of filtered signal to the state for the next call */

-    silk_memcpy( S->sFIR, &buf[ nSamplesIn ], S->FIR_Order * sizeof( opus_int32 ) );

+    silk_memcpy( S->sFIR.i32, &buf[ nSamplesIn ], S->FIR_Order * sizeof( opus_int32 ) );

 }

--- a/silk/resampler_structs.h

+++ b/silk/resampler_structs.h

@@ -37,7 +37,10 @@

 typedef struct _silk_resampler_state_struct{

     opus_int32       sIIR[ SILK_RESAMPLER_MAX_IIR_ORDER ]; /* this must be the first element of this struct */

-    opus_int32       sFIR[ SILK_RESAMPLER_MAX_FIR_ORDER ];

+    union{

+        opus_int32   i32[ SILK_RESAMPLER_MAX_FIR_ORDER ];

+        opus_int16   i16[ SILK_RESAMPLER_MAX_FIR_ORDER ];

+    }                sFIR;

     opus_int16       delayBuf[ 48 ];

     opus_int         resampler_function;

     opus_int         batchSize;