USB CDC NCM: Don't deref NULL in cdc_ncm_rx_fixup() and don't use uninitialized variable.

skb_clone() dynamically allocates memory and may fail. If it does it
returns NULL. This means we'll dereference a NULL pointer in
drivers/net/usb/cdc_ncm.c::cdc_ncm_rx_fixup().
As far as I can tell, the proper way to deal with this is simply to goto
the error label.

Furthermore gcc complains that 'skb' may be used uninitialized:
  drivers/net/usb/cdc_ncm.c: In function ‘cdc_ncm_rx_fixup’:
  drivers/net/usb/cdc_ncm.c:922:18: warning: ‘skb’ may be used uninitialized in this function
and I believe it is right. On the line where we
  pr_debug("invalid frame detected (ignored)" ...
we are using the local variable 'skb' but nothing has ever been assigned
to that variable yet. I believe the correct fix for that is to use
'skb_in' instead.

Signed-off-by: Jesper Juhl <jj@chaosbits.net>
Signed-off-by: David S. Miller <davem@davemloft.net>