qcow2-refcount: Sanitize refcount table entry

When reading the refcount table entry in get_refcount(), only bits which
are actually significant for the refcount block offset should be taken
into account.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NLaszlo Ersek <lersek@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/qcow2-refcount.c

@@ -96,7 +96,8 @@ static int get_refcount(BlockDriverState *bs, int64_t cluster_index)
     refcount_table_index = cluster_index >> (s->cluster_bits - REFCOUNT_SHIFT);
     if (refcount_table_index >= s->refcount_table_size)
         return 0;
-    refcount_block_offset = s->refcount_table[refcount_table_index];
+    refcount_block_offset =
+        s->refcount_table[refcount_table_index] & REFT_OFFSET_MASK;
     if (!refcount_block_offset)
         return 0;