ape: Use the proper variable type

Avoid an unsigned underflow.

Bug-Id: 1041122
CC: libav-stable@libav.org

libavformat/apetag.c

@@ -40,7 +40,7 @@ static int ape_tag_read_field(AVFormatContext *s)
 {
     AVIOContext *pb = s->pb;
     uint8_t key[1024], *value;
-    uint32_t size, flags;
+    int64_t size, flags;
     int i, c;
 
     size = avio_rl32(pb);  /* field size */