erspan: Check IFLA_GRE_ERSPAN_VER is set.

Add a check to make sure the IFLA_GRE_ERSPAN_VER is provided by users.

Fixes: f989d546 ("erspan: Add type I version 0 support.")
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NWilliam Tu <u9012063@gmail.com>
Reviewed-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_gre.c

@@ -1087,7 +1087,8 @@ static int erspan_validate(struct nlattr *tb[], struct nlattr *data[],
 	if (ret)
 		return ret;
 
-	if (nla_get_u8(data[IFLA_GRE_ERSPAN_VER]) == 0)
+	if (data[IFLA_GRE_ERSPAN_VER] &&
+	    nla_get_u8(data[IFLA_GRE_ERSPAN_VER]) == 0)
 		return 0;
 
 	/* ERSPAN type II/III should only have GRE sequence and key flag */