don't panic when injecting nil

inject.go

@@ -563,7 +563,7 @@ func parseTag(t string) (*tag, error) {
 }
 
 func isStructPtr(t reflect.Type) bool {
-	return t.Kind() == reflect.Ptr && t.Elem().Kind() == reflect.Struct
+	return t != nil && t.Kind() == reflect.Ptr && t.Elem().Kind() == reflect.Struct
 }
 
 func isNilOrZero(v reflect.Value, t reflect.Type) bool {

inject_test.go

@@ -994,3 +994,13 @@ func TestForSameNameButDifferentPackage(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestInjectNil(t *testing.T) {
+	var g inject.Graph
+	err := g.Provide(
+		&inject.Object{Value: nil},
+	)
+	if err == nil {
+		t.Fatal("expected error when injecting nil")
+	}
+}