fix(api): guard nil AccountID in getAdditionalInfoStonfi (panic on addr_none pool tokens)

[kesha-antonov]

What

getAdditionalInfoStonfi (pkg/api/emulation.go) panics with a nil-pointer
dereference when a STONfi pool's get_pool_data returns a token address of
addr_none (or addr_extern).

ton.AccountIDFromTlb returns (nil, nil) - a nil pointer and a nil error -
for AddrNone/AddrExtern, but the function only checks the error before
dereferencing the returned pointer:

t0, err0 := ton.AccountIDFromTlb(token0)
t1, err1 := ton.AccountIDFromTlb(token1)
if err1 != nil || err0 != nil {   // nil pointer not checked
    return
}
additionalInfo.STONfiPool = &core.STONfiPool{
    Token0: *t0,                  // panics if token0 == addr_none
    ...
    additionalInfo.SetJettonMaster(accountID, *master)  // panics if data.Jetton == addr_none

So emulating any trace that inspects such a contract panics the serving goroutine.

Why it's a clear bug, not an invariant

The NFT/jetton branch in the same function already guards exactly this pattern:

master, _ := ton.AccountIDFromTlb(data.Jetton)
if master == nil {
    continue
}

getAdditionalInfoStonfi simply omits the check on its three dereference sites.

Fix

Add the nil-pointer guards, mirroring the existing sibling branch (return early if
either token resolves to a nil AccountID; skip the jetton-master deref if it
does). One line per site, no behavior change for valid addresses.

Test

Adds TestStonfiNilAddressNoPanic (in pkg/api/): calls getAdditionalInfoStonfi
with addr_none token addresses and asserts it returns without panicking and
leaves STONfiPool unset. The test panics on the current code and passes with the
fix.

Reproduction (before the fix)

A get_pool_data (method id 81689) returning a STONfi-shaped stack whose
Token0Address/Token1Address slices hold the 2-bit addr_none prefix decodes
cleanly via abi.DecodeGetPoolData_StonfiResult and then panics in
getAdditionalInfoStonfi: runtime error: invalid memory address or nil pointer dereference at emulation.go:454.

[mr-tron]

lgtm

[Shemse1]

done ✔️