id3v2: reject APICs with size of 0

fixes ticket #1324
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/id3v2.c

@@ -488,7 +488,7 @@ static void read_apic(AVFormatContext *s, AVIOContext *pb, int taglen, char *tag
 
     apic->len   = taglen;
     apic->data  = av_malloc(taglen);
-    if (!apic->data || avio_read(pb, apic->data, taglen) != taglen)
+    if (!apic->data || !apic->len || avio_read(pb, apic->data, taglen) != taglen)
         goto fail;
 
     new_extra->tag    = "APIC";