Fix nil pointer dereference in x/distribution

Before checking Dec's value, one has to ensure the internal big.Int
pointer is not nil first.

Closes: #5621

CHANGELOG.md

@@ -51,6 +51,7 @@ balances or a single balance by denom when the `denom` query parameter is presen
 
 * (x/bank) [\#5531](https://github.com/cosmos/cosmos-sdk/issues/5531) Added missing amount event to MsgMultiSend, emitted for each output.
 * (client) [\#5618](https://github.com/cosmos/cosmos-sdk/pull/5618) Fix crash on the client when the verifier is not set. 
+* (x/distribution) [\#5620](https://github.com/cosmos/cosmos-sdk/pull/5620) Fix nil pointer deref in distribution tax/rewward validation helpers.
 
 ### State Machine Breaking
 

x/distribution/types/params.go

@@ -92,6 +92,9 @@ func validateCommunityTax(i interface{}) error {
 		return fmt.Errorf("invalid parameter type: %T", i)
 	}
 
+	if v.IsNil() {
+		return fmt.Errorf("community tax must be not nil")
+	}
 	if v.IsNegative() {
 		return fmt.Errorf("community tax must be positive: %s", v)
 	}
@@ -108,6 +111,9 @@ func validateBaseProposerReward(i interface{}) error {
 		return fmt.Errorf("invalid parameter type: %T", i)
 	}
 
+	if v.IsNil() {
+		return fmt.Errorf("base proposer reward must be not nil")
+	}
 	if v.IsNegative() {
 		return fmt.Errorf("base proposer reward must be positive: %s", v)
 	}
@@ -124,6 +130,9 @@ func validateBonusProposerReward(i interface{}) error {
 		return fmt.Errorf("invalid parameter type: %T", i)
 	}
 
+	if v.IsNil() {
+		return fmt.Errorf("bonus proposer reward must be not nil")
+	}
 	if v.IsNegative() {
 		return fmt.Errorf("bonus proposer reward must be positive: %s", v)
 	}

x/distribution/types/params_test.go

@@ -0,0 +1,33 @@
+package types
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	sdk "github.com/cosmos/cosmos-sdk/types"
+)
+
+func Test_validateAuxFuncs(t *testing.T) {
+	type args struct {
+		i interface{}
+	}
+	tests := []struct {
+		name    string
+		args    args
+		wantErr bool
+	}{
+		{"wrong type", args{10.5}, true},
+		{"nil Int pointer", args{sdk.Dec{}}, true},
+		{"negative", args{sdk.NewDec(-1)}, true},
+		{"one dec", args{sdk.NewDec(1)}, false},
+		{"two dec", args{sdk.NewDec(2)}, true},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.wantErr, validateCommunityTax(tt.args.i) != nil)
+			require.Equal(t, tt.wantErr, validateBaseProposerReward(tt.args.i) != nil)
+			require.Equal(t, tt.wantErr, validateBonusProposerReward(tt.args.i) != nil)
+		})
+	}
+}