Sanity check number of channels in setup.

Fixes #2335.

lib/vorbisenc.c

@@ -684,6 +684,7 @@ int vorbis_encode_setup_init(vorbis_info *vi){
   highlevel_encode_setup *hi=&ci->hi;
 
   if(ci==NULL)return(OV_EINVAL);
+  if(vi->channels<1||vi->channels>255)return(OV_EINVAL);
   if(!hi->impulse_block_p)i0=1;
 
   /* too low/high an ATH floater is nonsensical, but doesn't break anything */