netfilter: nf_nat_snmp_basic: add missing length checks in ASN.1 cbs

The generic ASN.1 decoder infrastructure doesn't guarantee that callbacks
will get as much data as they expect; callbacks have to check the `datalen`
parameter before looking at `data`. Make sure that snmp_version() and
snmp_helper() don't read/write beyond the end of the packet data.

(Also move the assignment to `pdata` down below the check to make it clear
that it isn't necessarily a pointer we can use before the `datalen` check.)

Fixes: cc2d5863

 ("netfilter: nf_nat_snmp_basic: use asn1 decoder library")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>