fix: correctly return constant expressions in functions

In the case of a function returning a single value, a constant
result could be ignored, and the function would return a zero value.

Fixes #889.

_test/const19.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+func get10Hours() time.Duration {
+	return 10 * time.Hour
+}
+
+func main() {
+	fmt.Println(get10Hours().String())
+}
+
+// Output:
+// 10h0m0s

interp/run.go

@@ -1869,7 +1869,10 @@ func _return(n *node) {
 	case 0:
 		n.exec = nil
 	case 1:
-		if child[0].kind == binaryExpr || isCall(child[0]) {
+		// This is an optimisation that is applied for binary expressions or function
+		// calls, but not for (binary) expressions involving const, as the values are not
+		// stored in the frame in that case.
+		if !child[0].rval.IsValid() && child[0].kind == binaryExpr || isCall(child[0]) {
 			n.exec = nil
 		} else {
 			v := values[0]