qcow2: fix new_blocks double-free in alloc_refcount_block()

Commit de82815d ("qcow2: Handle failure
for potentially large allocations") introduced a double-free of
new_blocks in the alloc_refcount_block() error path.

The qemu-iotests qcow2 026 test case was failing because qemu-io
segfaulted.

Make sure new_blocks is NULL after we free it the first time.
Reviewed-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/qcow2-refcount.c

@@ -381,6 +381,7 @@ static int alloc_refcount_block(BlockDriverState *bs,
     ret = bdrv_pwrite_sync(bs->file, meta_offset, new_blocks,
         blocks_clusters * s->cluster_size);
     g_free(new_blocks);
+    new_blocks = NULL;
     if (ret < 0) {
         goto fail_table;
     }