oggenc: check return value of av_mallocz()

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/oggenc.c

@@ -427,6 +427,9 @@ static int ogg_write_header(AVFormatContext *s)
             return -1;
         }
         oggstream = av_mallocz(sizeof(*oggstream));
+        if (!oggstream)
+            return AVERROR(ENOMEM);
+
         oggstream->page.stream_index = i;
 
         if (!(st->codec->flags & CODEC_FLAG_BITEXACT))