ipv4/fib: don't warn when primary address is missing if in_dev is dead

After commit fbd40ea0 ("ipv4: Don't do expensive useless work
during inetdev destroy.") when deleting an interface,
fib_del_ifaddr() can be executed without any primary address
present on the dead interface.

The above is safe, but triggers some "bug: prim == NULL" warnings.

This commit avoids warning if the in_dev is dead

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>