qcow2: Check qcow2_alloc_clusters_at() return value

When using qcow2_alloc_clusters_at(), the cluster allocation code
checked the wrong variable for an error code.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2-cluster.c

@@ -762,7 +762,6 @@ static int do_alloc_cluster_offset(BlockDriverState *bs, uint64_t guest_offset,
     uint64_t *host_offset, unsigned int *nb_clusters)
 {
     BDRVQcowState *s = bs->opaque;
-    int64_t cluster_offset;
     QCowL2Meta *old_alloc;
 
     trace_qcow2_do_alloc_clusters_offset(qemu_coroutine_self(), guest_offset,
@@ -808,17 +807,21 @@ static int do_alloc_cluster_offset(BlockDriverState *bs, uint64_t guest_offset,
     /* Allocate new clusters */
     trace_qcow2_cluster_alloc_phys(qemu_coroutine_self());
     if (*host_offset == 0) {
-        cluster_offset = qcow2_alloc_clusters(bs, *nb_clusters * s->cluster_size);
+        int64_t cluster_offset =
+            qcow2_alloc_clusters(bs, *nb_clusters * s->cluster_size);
+        if (cluster_offset < 0) {
+            return cluster_offset;
+        }
+        *host_offset = cluster_offset;
+        return 0;
     } else {
-        cluster_offset = *host_offset;
-        *nb_clusters = qcow2_alloc_clusters_at(bs, cluster_offset, *nb_clusters);
-    }
-
-    if (cluster_offset < 0) {
-        return cluster_offset;
+        int ret = qcow2_alloc_clusters_at(bs, *host_offset, *nb_clusters);
+        if (ret < 0) {
+            return ret;
+        }
+        *nb_clusters = ret;
+        return 0;
     }
-    *host_offset = cluster_offset;
-    return 0;
 }
 
 /*