avformat/asfenc: Check for index_ptr allocation failure

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/asfenc.c

@@ -652,6 +652,8 @@ static int asf_write_header(AVFormatContext *s)
     asf->nb_packets = 0;
 
     asf->index_ptr             = av_malloc(sizeof(ASFIndex) * ASF_INDEX_BLOCK);
+    if (!asf->index_ptr)
+        return AVERROR(ENOMEM);
     asf->nb_index_memory_alloc = ASF_INDEX_BLOCK;
     asf->maximum_packet        = 0;