soxdec: check av_malloc return.

Bug found by: durandal_1707
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/soxdec.c

@@ -98,6 +98,8 @@ static int sox_read_header(AVFormatContext *s,
 
     if (comment_size && comment_size < UINT_MAX) {
         char *comment = av_malloc(comment_size+1);
+        if(!comment)
+            return AVERROR(ENOMEM);
         if (avio_read(pb, comment, comment_size) != comment_size) {
             av_freep(&comment);
             return AVERROR(EIO);