l2: fix null deref in l2_to_bvi()

l2: fix null deref in l2_to_bvi()

Type: fix

Static analysis identified a possible null pointer dereference. It
was introduced by a recent patch which expanded the DMAC comparison
on inbound packets on a BVI interface to include any secondary MAC
addresses which were added to an interface.

Check if the pointer is null before dereferencing.

Change-Id: Ic2afe2b062eda32977e05bf3f98d82c1fe64620c
Signed-off-by: Matthew Smith <mgsmith@netgate.com>