core: fixes some more panics (#2137)

Fixes some more panics found by beacon fuzz.

category: misc
ticket: #1962

core/fetcher/fetcher.go

@@ -132,6 +132,8 @@ func (f *Fetcher) fetchAttesterData(ctx context.Context, slot int64, defSet core
 			eth2AttData, err = f.eth2Cl.AttestationData(ctx, eth2p0.Slot(uint64(slot)), commIdx)
 			if err != nil {
 				return nil, err
+			} else if eth2AttData == nil {
+				return nil, errors.New("attestation data cannot be nil")
 			}
 
 			dataByCommIdx[commIdx] = eth2AttData

core/tracker/inclusion.go

@@ -367,6 +367,10 @@ func (a *InclusionChecker) checkBlock(ctx context.Context, slot int64) error {
 			return errors.New("invalid attestation")
 		}
 
+		if att.Data.Target == nil || att.Data.Source == nil {
+			return errors.New("invalid attestation data checkpoint")
+		}
+
 		root, err := att.Data.HashTreeRoot()
 		if err != nil {
 			return errors.Wrap(err, "hash attestation")

testutil/beaconmock/beaconmock_fuzz.go

@@ -68,12 +68,20 @@ func WithBeaconMockFuzzer() Option {
 
 					// Return expected attester duties
 					vals := getValidators()
+					if vals == nil {
+						return
+					}
 					var resp []*eth2v1.AttesterDuty
 					for _, vIdx := range indices {
 						var duty eth2v1.AttesterDuty
 						c.Fuzz(&duty)
 
-						duty.PubKey = vals[vIdx].Validator.PublicKey
+						val, ok := vals[vIdx]
+						if !ok {
+							continue
+						}
+
+						duty.PubKey = val.Validator.PublicKey
 						duty.ValidatorIndex = vIdx
 						duty.Slot = eth2p0.Slot(int(epoch*16) + c.Intn(16))
 						resp = append(resp, &duty)