Fix for [oCERT-2008-004] multiple speex implementations insufficient boundary...

Fix for [oCERT-2008-004] multiple speex implementations insufficient boundary checks. Patch by Eren Türkay
(cherry picked from commit 8060b345)

modules/codec/speex.c

@@ -332,7 +332,7 @@ static int ProcessInitialHeader( decoder_t *p_dec, ogg_packet *p_oggpacket )
         msg_Err( p_dec, "cannot read Speex header" );
         return VLC_EGENERIC;
     }
-    if( p_header->mode >= SPEEX_NB_MODES )
+    if( p_header->mode >= SPEEX_NB_MODES || p_header->mode < 0 )
     {
         msg_Err( p_dec, "mode number %d does not (yet/any longer) exist in "
                  "this version of libspeex.", p_header->mode );