iavf: Fix bad page state

Fix bad page state, free inappropriate page in handling dummy
descriptor. iavf_build_skb now has to check not only if rx_buffer is
NULL but also if size is zero, same thing in iavf_clean_rx_irq.
Without this patch driver would free page that will be used
by napi_build_skb.

Fixes: a9f49e00 ("iavf: Fix handling of dummy receive descriptors")
Signed-off-by: Norbert Zulinski <norbertx.zulinski@intel.com>
Signed-off-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>