skiptags: only check the extra footer size for APEv2

APEv1 should have flags set to 0 (no footer), but let's not assume writers set
this properly.

https://mutagen-specs.readthedocs.io/en/latest/apev2/apev2.html

modules/stream_filter/skiptags.c

@@ -98,15 +98,18 @@ static uint_fast32_t SkipAPETag(stream_t *s)
 
     uint_fast32_t size = GetDWLE(peek + 12);
 
-    uint_fast32_t flags = GetDWLE(peek + 16);
-    if ((flags & (1u << 29)) == 0)
-        return 0;
-
-    if (flags & (1u << 30))
+    if (version >= 2000)
     {
-        if (size > UINT32_MAX - 32u)
-            return 0; /* impossibly long tag */
-        size += 32;
+        uint_fast32_t flags = GetDWLE(peek + 16);
+        if ((flags & (1u << 29)) == 0)
+            return 0;
+
+        if (flags & (1u << 30))
+        {
+            if (size > UINT32_MAX - 32u)
+                return 0; /* impossibly long tag */
+            size += 32;
+        }
     }
 
     msg_Dbg(s, "AP2 v%"PRIuFAST32" tag found, "