migration/qemu-file: remove check on writev_buffer in qemu_put_compression_data

The check of writev_buffer is in qemu_fflush, which means it is not
harmful if it is NULL.

And removing it will make the code consistent since all other
add_to_iovec() is called without the check.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20190911132839.23336-2-richard.weiyang@gmail.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>