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.ru
Signed-off-by: Paolo Abeni <pabeni@redhat.com>