interp: wrap source functions when used as input parameters.

It allows to use interpreter functions as parameters in the runtime,
even for defered callbacks, or when passed as empty interfaces,
as for runtime.SetFinalizer.

Fixes traefik#1503

_test/fun28.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"runtime"
+)
+
+type T struct {
+	name string
+}
+
+func finalize(t *T) { println("finalize") }
+
+func newT() *T {
+	t := new(T)
+	runtime.SetFinalizer(t, finalize)
+	return t
+}
+
+func main() {
+	t := newT()
+	println(t != nil)
+}
+
+// Output:
+// true

interp/run.go

@@ -1561,6 +1561,8 @@ func callBin(n *node) {
 				values = append(values, genValue(c))
 			case isInterfaceSrc(c.typ):
 				values = append(values, genValueInterfaceValue(c))
+			case isFuncSrc(c.typ):
+				values = append(values, genFunctionWrapper(c))
 			case c.typ.cat == arrayT || c.typ.cat == variadicT:
 				if isEmptyInterface(c.typ.val) {
 					values = append(values, genValueArray(c))