mlxfw: fix null-ptr-deref in mlxfw_mfa2_tlv_next()

Function mlxfw_mfa2_tlv_multi_get() returns NULL if 'tlv' in
question does not pass checks in mlxfw_mfa2_tlv_payload_get(). This
behaviour may lead to NULL pointer dereference in 'multi->total_len'.
Fix this issue by testing mlxfw_mfa2_tlv_multi_get()'s return value
against NULL.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 410ed13c ("Add the mlxfw module for Mellanox firmware flash process")
Co-developed-by: Natalia Petrova <n.petrova@fintech.ru>
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/20230417120718.52325-1-n.zhandarovich@fintech.ruSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/mellanox/mlxfw/mlxfw_mfa2_tlv_multi.c

@@ -31,6 +31,8 @@ mlxfw_mfa2_tlv_next(const struct mlxfw_mfa2_file *mfa2_file,
 
 	if (tlv->type == MLXFW_MFA2_TLV_MULTI_PART) {
 		multi = mlxfw_mfa2_tlv_multi_get(mfa2_file, tlv);
+		if (!multi)
+			return NULL;
 		tlv_len = NLA_ALIGN(tlv_len + be16_to_cpu(multi->total_len));
 	}