Fix excessive stack usage when calling vorbis_analysis_wrote with l…

…ots of samples

`vorbis_analysis_wrote` increments `v->pcm_current` by `vals`, and this
incremented value can be used by `_preextrapolate_helper` right after to
allocate a float array in the stack `v->pcm_current` positions large.
Clearly, since `alloca` does not check that there is enough stack space
available to satisfy the allocation request, this can lead to a stack
overflow and memory corruption, which at best have no effect, more
likely cause segmentation faults, and at worst introduce security risks.

The documentation for `vorbis_analysis_buffer` and
`vorbis_analysis_wrote` does not specify a maximum value for `vals`. It
states that "1024 is a reasonable choice", but callers are free to use
larger or smaller counts as they wish. Therefore, `libvorbis` not
handling this case is undesirable behavior.

To better handle this case without throwing the performance benefits of
`alloca` out the window, let's check whether the allocation would exceed
256 KiB (an estimate for the minimum stack space available is 1 MiB,
which is [the default on Windows
platforms](https://learn.microsoft.com/en-us/windows/win32/procthread/thread-stack-size)),
and if so fall back to a heap allocated array. The heap array that may
be allocated for this purpose is freed when `vorbis_dsp_clear` is
called. `_preextrapolate_helper` takes neglible execution time
compared to the encoding process for usual sample block sizes, though.

Signed-off-by: Alejandro González <me@alegon.dev>

include/vorbis/codec.h

@@ -61,6 +61,7 @@ typedef struct vorbis_dsp_state{
 
   float **pcm;
   float **pcmret;
+  float *preextrapolate_work;
   int      pcm_storage;
   int      pcm_current;
   int      pcm_returned;

lib/block.c

@@ -372,6 +372,7 @@ void vorbis_dsp_clear(vorbis_dsp_state *v){
           if(v->pcm[i])_ogg_free(v->pcm[i]);
       _ogg_free(v->pcm);
       if(v->pcmret)_ogg_free(v->pcmret);
+      if(v->preextrapolate_work)_ogg_free(v->preextrapolate_work);
     }
 
     if(b){
@@ -417,11 +418,19 @@ static void _preextrapolate_helper(vorbis_dsp_state *v){
   int i;
   int order=16;
   float *lpc=alloca(order*sizeof(*lpc));
-  float *work=alloca(v->pcm_current*sizeof(*work));
+  float *work;
+  int workbuf=v->pcm_current*sizeof(*work);
   long j;
   v->preextrapolate=1;
 
-  if(v->pcm_current-v->centerW>order*2){ /* safety */
+  if(workbuf<256*1024)
+    work=alloca(workbuf);
+  else
+    /* workbuf is too big to safely allocate on the stack */
+    v->preextrapolate_work=_ogg_realloc(v->preextrapolate_work,workbuf);
+    work=v->preextrapolate_work;
+
+  if(v->pcm_current-v->centerW>order*2 && work){ /* safety */
     for(i=0;i<v->vi->channels;i++){
       /* need to run the extrapolation in reverse! */
       for(j=0;j<v->pcm_current;j++)