staging: rtl8192u: Add null check in rtl8192_usb_initendpoints

There is an allocation for priv->rx_urb[16] has no null check,
which may lead to a null pointer dereference.

Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Link: https://lore.kernel.org/r/20201226080258.6576-1-dinghao.liu@zju.edu.cn


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>