qcow2: Fix alloc_clusters_noref() overflow detection

If the very first allocation has a length of 0, the free_cluster_index
is still 0 after the for loop, which means that subtracting one from it
will underflow and signal an invalid range of clusters by returning
-EFBIG. However, there is no such range, as its length is 0.

Fix this by preventing underflows on free_cluster_index during the
check.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/qcow2-refcount.c

@@ -656,7 +656,9 @@ retry:
 
     /* Make sure that all offsets in the "allocated" range are representable
      * in an int64_t */
-    if (s->free_cluster_index - 1 > (INT64_MAX >> s->cluster_bits)) {
+    if (s->free_cluster_index > 0 &&
+        s->free_cluster_index - 1 > (INT64_MAX >> s->cluster_bits))
+    {
         return -EFBIG;
     }