Fix crasher on history underflow.

From fuzz testing: getHistory() wasn't being bounds-checked, and so
would crash if an @-token referenced before the first evaluation.
Reproducing this requires either serialized testing or clearing the
history, so the ClearHistory() needs to be called first to reproduce it
reliably.

compute/compute.go

@@ -34,7 +34,11 @@ ScanLoop:
 		_, tok, lit := s.Scan()
 
 		if lit != "@" && back > -1 && len(resHistory) > 0 {
-			floats.Push(getHistory(back))
+			if hval, err := getHistory(back); err != nil {
+				return 0, err
+			} else {
+				floats.Push(hval)
+			}
 			if prev == token.RPAREN || constants.IsConstant(prev.String()) {
 				evalUnprecedenced("*", ops, floats)
 			}
@@ -189,8 +193,11 @@ func parseOperator(lit string) *operators.Operator {
 	return operators.FindOperatorFromString(lit)
 }
 
-func getHistory(back int) float64 {
-	return resHistory[back]
+func getHistory(back int) (float64, error) {
+	if back >= len(resHistory) {
+		return 0, errors.New("History underflow")
+	}
+	return resHistory[back], nil
 }
 
 func pushHistory(res float64) {

compute/compute_test.go

@@ -50,6 +50,34 @@ func TestEvaluate(t *testing.T) {
 	}
 }
 
+func TestEvaluateInvalid(t *testing.T) {
+	tests := [][]string{
+		{"/"},
+		{"1/"},
+		{"1("},
+		{")("},
+		{"(()"},
+		{"@"},
+		{"@@"},
+		{"0", "@@"},
+		{"0", "@@@"},
+		{"@@\xa6"},
+	}
+	for i, series := range tests {
+		ClearHistory()
+		var fail error
+		for _, expr := range series {
+			if _, err := Evaluate(expr); err != nil {
+				fail = err
+				break
+			}
+		}
+		if fail == nil {
+			t.Errorf("case %d: expected error, finished successfully", i)
+		}
+	}
+}
+
 func BenchmarkEvaluate(b *testing.B) {
 	tests := []string{
 		"π",