usbnet: make sure no NULL pointer is passed through

commit 6c22fce07c97f765af1808ec3be007847e0b47d1 upstream.

Coverity reports:

	** CID 751368:  Null pointer dereferences  (FORWARD_NULL)
	/drivers/net/usb/usbnet.c: 1925 in __usbnet_read_cmd()

	________________________________________________________________________________________________________
	*** CID 751368:  Null pointer dereferences  (FORWARD_NULL)
	/drivers/net/usb/usbnet.c: 1925 in __usbnet_read_cmd()
	1919     EXPORT_SYMBOL(usbnet_link_change);
	1920
	1921     /*-------------------------------------------------------------------------*/
	1922     static int __usbnet_read_cmd(struct usbnet *dev, u8 cmd, u8 reqtype,
	1923                                 u16 value, u16 index, void *data, u16 size)
	1924     {
	>>>     CID 751368:  Null pointer dereferences  (FORWARD_NULL)
	>>>     Assigning: "buf" = "NULL".
	1925            void *buf = NULL;
	1926            int err = -ENOMEM;
	1927
	1928            netdev_dbg(dev->net, "usbnet_read_cmd cmd=0x%02x reqtype=%02x"
	1929                       " value=0x%04x index=0x%04x size=%d\n",
	1930                       cmd, reqtype, value, index, size);

	** CID 751370:  Null pointer dereferences  (FORWARD_NULL)
	/drivers/net/usb/usbnet.c: 1952 in __usbnet_write_cmd()

	________________________________________________________________________________________________________
	*** CID 751370:  Null pointer dereferences  (FORWARD_NULL)
	/drivers/net/usb/usbnet.c: 1952 in __usbnet_write_cmd()
	1946     }
	1947
	1948     static int __usbnet_write_cmd(struct usbnet *dev, u8 cmd, u8 reqtype,
	1949                                  u16 value, u16 index, const void *data,
	1950                                  u16 size)
	1951     {
	>>>     CID 751370:  Null pointer dereferences  (FORWARD_NULL)
	>>>     Assigning: "buf" = "NULL".
	1952            void *buf = NULL;
	1953            int err = -ENOMEM;
	1954
	1955            netdev_dbg(dev->net, "usbnet_write_cmd cmd=0x%02x reqtype=%02x"
	1956                       " value=0x%04x index=0x%04x size=%d\n",
	1957                       cmd, reqtype, value, index, size);

	** CID 1325026:  Null pointer dereferences  (FORWARD_NULL)
	/drivers/net/usb/ch9200.c: 143 in control_write()

It is valid to offer commands without a buffer, but then you need a size
of zero. This should actually be checked.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>