fixed issues found by go-fuzz

.gitignore

@@ -0,0 +1,3 @@
+fuzz
+fuzz.go
+filter-fuzz.zip

filter.go

@@ -102,6 +102,9 @@ func NewQuery(s string, l Logic) (q *Query, err error) {
 					switch s[2] {
 					case asc, desc:
 						q.s.fields = strings.Split(s[1], ".")
+						if err = checkFields(q.s.fields, v); err != nil {
+							return
+						}
 						if len(q.s.fields) > 0 {
 							if s[2] == desc {
 								q.s.desc = true
@@ -129,8 +132,12 @@ func NewQuery(s string, l Logic) (q *Query, err error) {
 			}
 			qm[m[0]] = struct{}{}
 		}
+		mf := strings.Split(m[0], ".")
+		if err = checkFields(mf, m[0]); err != nil {
+			return
+		}
 		q.m = append(q.m, match{
-			strings.Split(m[0], "."),
+			mf,
 			matchFunc(m[1]),
 			boolFunc(m[1]),
 		})
@@ -191,9 +198,9 @@ func (q *Query) match(v reflect.Value) (ok bool) {
 		if fl == 0 {
 			continue
 		}
-		val := v.FieldByName(qv.fields[0])
-		for j := 1; j < fl; j++ {
-			val = val.FieldByName(qv.fields[j])
+		val := walkFields(v.FieldByName(qv.fields[0]), qv.fields[1:])
+		if val.Kind() == reflect.Invalid {
+			continue
 		}
 		s, b, err := getValue(val)
 		if err != nil {
@@ -239,6 +246,9 @@ func (q Query) sort(f Filter) {
 		return
 	}
 	l := makeLess(f, q.s.fields)
+	if l == nil {
+		return
+	}
 	if q.s.desc {
 		l = reverse(l)
 	}

helper.go

@@ -12,6 +12,15 @@ const (
 	anys = string(any)
 )
 
+func checkFields(s []string, val string) error {
+	for _, v := range s {
+		if v == "" {
+			return errors.New("invalid subfield: " + val)
+		}
+	}
+	return nil
+}
+
 type less func(i, j int) bool
 
 func reverse(f less) less {
@@ -20,12 +29,18 @@ func reverse(f less) less {
 	}
 }
 
-func index(f Filter, i int, fields []string) (val reflect.Value) {
-	val = reflect.ValueOf(f.Index(i)).FieldByName(fields[0])
-	for j := 1; j < len(fields); j++ {
-		val = val.FieldByName(fields[j])
+func walkFields(v reflect.Value, fields []string) reflect.Value {
+	for i := 0; i < len(fields); i++ {
+		if v.Kind() != reflect.Struct {
+			return reflect.ValueOf(nil)
+		}
+		v = v.FieldByName(fields[i])
 	}
-	return
+	return v
+}
+
+func index(f Filter, i int, fields []string) (val reflect.Value) {
+	return walkFields(reflect.ValueOf(f.Index(i)).FieldByName(fields[0]), fields[1:])
 }
 
 func makeLess(f Filter, fields []string) less {