iso9660: Fix volume descriptor start offset

Volume descriptors start at 32KiB, which is 16(!) blocks.
diff --git a/src/filo-fs-iso9660.adb b/src/filo-fs-iso9660.adb
index 03f1abb..e4bc3df 100644
--- a/src/filo-fs-iso9660.adb
+++ b/src/filo-fs-iso9660.adb
@@ -61,7 +61,7 @@
       Static : Mount_State renames State.Static;
 
       Volume_Descriptor : Buffer_Type (0 .. D_Ver);
-      Block : FSBlock := 32;
+      Block : FSBlock := 16;
    begin
       loop
          pragma Loop_Invariant (Block < 1024);