avformat/hnm: Check for extradata allocation failure

and also add padding to it; moreover, don't use memcpy to write one byte
to extradata.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/hnm.c

@@ -70,6 +70,7 @@ static int hnm_read_header(AVFormatContext *s)
     Hnm4DemuxContext *hnm = s->priv_data;
     AVIOContext *pb = s->pb;
     AVStream *vst;
+    int ret;
 
     /* default context members */
     hnm->pts = 0;
@@ -113,10 +114,10 @@ static int hnm_read_header(AVFormatContext *s)
     vst->codecpar->codec_tag  = 0;
     vst->codecpar->width      = hnm->width;
     vst->codecpar->height     = hnm->height;
-    vst->codecpar->extradata  = av_mallocz(1);
+    if ((ret = ff_alloc_extradata(vst->codecpar, 1)) < 0)
+        return ret;
 
-    vst->codecpar->extradata_size = 1;
-    memcpy(vst->codecpar->extradata, &hnm->version, 1);
+    vst->codecpar->extradata[0] = hnm->version;
 
     vst->start_time = 0;