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: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>