Return NULL if we got unexpected value from skb_trim_rcsum()
in ksz_common_rcv()
Found by Linux Verification Center (linuxtesting.org) with SVACE.
Fixes: bafe9ba7d908 ("net: dsa: ksz: Factor out common tag code")
Signed-off-by: Artem Chernyshev <artem.chernyshev@xxxxxxxxxxx>
Reviewed-by: Vladimir Oltean <olteanv@xxxxxxxxx>