Fixed Kecakke256 computation panics in some transaction state

The EVM does memory expansion **after** notifying us about OnOpcode which we use to compute Keccak256 pre-images now. This creates problem when we want to retrieve the preimage data because the memory is not expanded yet but in the EVM is going to work because the memory is going to be expanded before the operation is actually executed so the memory will be of the correct size.

In this situation, we must pad with zeroes when the memory is not big enough.

eth/tracers/firehose.go

@@ -2274,9 +2274,17 @@ func (m Memory) GetPtr(offset, size int64) []byte {
 		return nil
 	}
 
-	if len(m) > int(offset) {
+	if len(m) >= (int(offset) + int(size)) {
 		return m[offset : offset+size]
 	}
 
-	return nil
+	// The EVM does memory expansion **after** notifying us about OnOpcode which we use
+	// to compute Keccak256 pre-images now. This creates problem when we want to retrieve
+	// the preimage data because the memory is not expanded yet but in the EVM is going to
+	// work because the memory is going to be expanded before the operation is actually
+	// executed so the memory will be of the correct size.
+	//
+	// In this situtation, we must pad with zeroes when the memory is not big enough.
+	reminder := m[offset:]
+	return append(reminder, make([]byte, int(size)-len(reminder))...)
 }

eth/tracers/firehose_test.go

@@ -468,3 +468,25 @@ func blockEvent(height uint64) tracing.BlockEvent {
 		TD: b(1),
 	}
 }
+
+func TestMemory_GetPtr(t *testing.T) {
+	type args struct {
+		offset int64
+		size   int64
+	}
+	tests := []struct {
+		name string
+		m    Memory
+		args args
+		want []byte
+	}{
+		{"memory is just a bit too small", Memory([]byte{1, 2, 3}), args{0, 4}, []byte{1, 2, 3, 0}},
+		{"memory is flushed with request", Memory([]byte{1, 2, 3, 4}), args{0, 4}, []byte{1, 2, 3, 4}},
+		{"memory is just a bit too big", Memory([]byte{1, 2, 3, 4, 5}), args{0, 4}, []byte{1, 2, 3, 4}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.want, tt.m.GetPtr(tt.args.offset, tt.args.size))
+		})
+	}
+}