ax25: zero length frame filtering in AX25

In previous commit 244f46ae
was introduced a zero length frame filter for ROSE protocole.
This patch has the same purpose at AX25 frame level for the same
reason. Empty frames have no meaning in AX25 protocole.

Signed-off-by: Bernard Pidoux <f6bvp@amsat.org>
Signed-off-by: David S. Miller <davem@davemloft.net>