net: sun: sungem: rix a possible null dereference

The function gem_begin_auto_negotiation dereference
the pointer ep before testing if it's null. This
patch add a check on ep before dereferencing it.

Fixes: 92552fdd ("net: sun: sungem: use new api
ethtool_{get|set}_link_ksettings")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/sungem.c

@@ -1259,8 +1259,9 @@ static void gem_begin_auto_negotiation(struct gem *gp,
 	int duplex;
 	u32 advertising;
 
-	ethtool_convert_link_mode_to_legacy_u32(&advertising,
-						ep->link_modes.advertising);
+	if (ep)
+		ethtool_convert_link_mode_to_legacy_u32(
+			&advertising, ep->link_modes.advertising);
 
 	if (gp->phy_type != phy_mii_mdio0 &&
      	    gp->phy_type != phy_mii_mdio1)