smacker: pad the extradata allocation

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/smacker.c

@@ -210,7 +210,8 @@ static int smacker_read_header(AVFormatContext *s)
 
 
     /* load trees to extradata, they will be unpacked by decoder */
-    st->codec->extradata = av_malloc(smk->treesize + 16);
+    st->codec->extradata = av_mallocz(smk->treesize + 16 +
+                                      FF_INPUT_BUFFER_PADDING_SIZE);
     st->codec->extradata_size = smk->treesize + 16;
     if(!st->codec->extradata){
         av_log(s, AV_LOG_ERROR, "Cannot allocate %i bytes of extradata\n", smk->treesize + 16);