avformat/mov: set correct error code in mov_read_custom

In function mov_read_custom(), it returns 0 on the path that av_malloc()
returns a NULL pointer. 0 indicates success. An error code should be
assigned to ret.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -4176,8 +4176,10 @@ static int mov_read_custom(MOVContext *c, AVIOContext *pb, MOVAtom atom)
             break;
 
         *p = av_malloc(len + 1);
-        if (!*p)
+        if (!*p) {
+            ret = AVERROR(ENOMEM);
             break;
+        }
         ret = ffio_read_size(pb, *p, len);
         if (ret < 0) {
             av_freep(p);