Fix data race around accessing custom validators

Because I already introduced a BC break, why not add another? The last
build mysteriously failed due to a data race that happens when accessing
the custom validators. You need to use the new `Get` and `Set` methods
on the `CustomTypeTagMap` struct which protects its private validator
type map with a mutex.

types.go

@@ -3,6 +3,7 @@ package govalidator
 import (
 	"reflect"
 	"regexp"
+	"sync"
 )
 
 // Validator is a wrapper for a validator function that returns bool and accepts string.
@@ -39,10 +40,29 @@ var ParamTagRegexMap = map[string]*regexp.Regexp{
 	"matches":      regexp.MustCompile(`matches\(([^)]+)\)`),
 }
 
+type customTypeTagMap struct {
+	validators map[string]CustomTypeValidator
+
+	sync.RWMutex
+}
+
+func (tm *customTypeTagMap) Get(name string) (CustomTypeValidator, bool) {
+	tm.RLock()
+	defer tm.RUnlock()
+	v, ok := tm.validators[name]
+	return v, ok
+}
+
+func (tm *customTypeTagMap) Set(name string, ctv CustomTypeValidator) {
+	tm.Lock()
+	defer tm.Unlock()
+	tm.validators[name] = ctv
+}
+
 // CustomTypeTagMap is a map of functions that can be used as tags for ValidateStruct function.
 // Use this to validate compound or custom types that need to be handled as a whole, e.g.
 // `type UUID [16]byte` (this would be handled as an array of bytes).
-var CustomTypeTagMap = map[string]CustomTypeValidator{}
+var CustomTypeTagMap = &customTypeTagMap{validators: make(map[string]CustomTypeValidator)}
 
 // TagMap is a map of functions, that can be used as tags for ValidateStruct function.
 var TagMap = map[string]Validator{

validator.go

@@ -708,7 +708,7 @@ func typeCheck(v reflect.Value, t reflect.StructField, o reflect.Value) (bool, e
 		if ok := isValidTag(tagOpts[0]); !ok {
 			continue
 		}
-		if validatefunc, ok := CustomTypeTagMap[tagOpts[0]]; ok {
+		if validatefunc, ok := CustomTypeTagMap.Get(tagOpts[0]); ok {
 			customTypeValidatorsExist = true
 			if result := validatefunc(v.Interface(), o.Interface()); !result {
 				if len(tagOpts) == 2 {

validator_test.go

@@ -1933,7 +1933,7 @@ func TestStructWithCustomByteArray(t *testing.T) {
 	t.Parallel()
 
 	// add our custom byte array validator that fails when the byte array is pristine (all zeroes)
-	CustomTypeTagMap["customByteArrayValidator"] = CustomTypeValidator(func(i interface{}, o interface{}) bool {
+	CustomTypeTagMap.Set("customByteArrayValidator", CustomTypeValidator(func(i interface{}, o interface{}) bool {
 		switch v := o.(type) {
 		case StructWithCustomByteArray:
 			if len(v.Email) > 0 {
@@ -1954,14 +1954,14 @@ func TestStructWithCustomByteArray(t *testing.T) {
 			}
 		}
 		return false
-	})
-	CustomTypeTagMap["customMinLengthValidator"] = CustomTypeValidator(func(i interface{}, o interface{}) bool {
+	}))
+	CustomTypeTagMap.Set("customMinLengthValidator", CustomTypeValidator(func(i interface{}, o interface{}) bool {
 		switch v := o.(type) {
 		case StructWithCustomByteArray:
 			return len(v.ID) >= v.CustomMinLength
 		}
 		return false
-	})
+	}))
 	testCustomByteArray := CustomByteArray{'1', '2', '3', '4', '5', '6'}
 	var tests = []struct {
 		param    StructWithCustomByteArray