apetag: propagate errors.

Fixes crashes if reading the tag value fails.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavformat/apetag.c

@@ -75,6 +75,8 @@ static int ape_tag_read_field(AVFormatContext *s)
         if (!value)
             return AVERROR(ENOMEM);
         c = avio_read(pb, value, size);
+        if (c < 0)
+            return c;
         value[c] = 0;
         av_dict_set(&s->metadata, key, value, AV_DICT_DONT_STRDUP_VAL);
     }