[PPP]: remove redundant NULL pointer checks before kfree & vfree

kfree() and vfree() can both deal with NULL pointers. This patch removes 
redundant NULL pointer checks from the ppp code in drivers/net/

Signed-off-by: Jesper Juhl <juhl-lkml@dif.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>