iavf: fix dereference of null rx_buffer pointer

A recent commit efa14c39 ("iavf: allow null RX descriptors") added
a null pointer sanity check on rx_buffer, however, rx_buffer is being
dereferenced before that check, which implies a null pointer dereference
bug can potentially occur.  Fix this by only dereferencing rx_buffer
until after the null pointer check.

Addresses-Coverity: ("Dereference before null check")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>