interp: fix generic check on nil function

Related to issue traefik/traefik#9231

Author: mvertes

_test/gen10.go

@@ -0,0 +1,12 @@
+package main
+
+func genFunc() (f func()) {
+	return f
+}
+
+func main() {
+	println(genFunc() == nil)
+}
+
+// Output:
+// true

interp/type.go

@@ -2213,7 +2213,7 @@ func isEmptyInterface(t *itype) bool {
 }
 
 func isGeneric(t *itype) bool {
-	return t.cat == funcT && t.node != nil && len(t.node.child[0].child) > 0
+	return t.cat == funcT && t.node != nil && len(t.node.child) > 0 && len(t.node.child[0].child) > 0
 }
 
 func isFuncSrc(t *itype) bool {