usbnet: make sure no NULL pointer is passed through

Coverity reports:

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

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*** 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()

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*** 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>