fix issue in package gjson for interface assertion of nil Json

encoding/gjson/gjson_api.go

@@ -17,6 +17,9 @@ import (
 
 // Interface returns the json value.
 func (j *Json) Interface() interface{} {
+	if j == nil {
+		return nil
+	}
 	j.mu.RLock()
 	defer j.mu.RUnlock()
 	return *(j.p)
@@ -29,6 +32,9 @@ func (j *Json) Var() *gvar.Var {
 
 // IsNil checks whether the value pointed by `j` is nil.
 func (j *Json) IsNil() bool {
+	if j == nil {
+		return true
+	}
 	j.mu.RLock()
 	defer j.mu.RUnlock()
 	return j.p == nil || *(j.p) == nil
@@ -43,6 +49,9 @@ func (j *Json) IsNil() bool {
 //
 // It returns a default value specified by `def` if value for `pattern` is not found.
 func (j *Json) Get(pattern string, def ...interface{}) *gvar.Var {
+	if j == nil {
+		return nil
+	}
 	j.mu.RLock()
 	defer j.mu.RUnlock()
 
@@ -196,6 +205,9 @@ func (j *Json) Scan(pointer interface{}, mapping ...map[string]string) error {
 
 // Dump prints current Json object with more manually readable.
 func (j *Json) Dump() {
+	if j == nil {
+		return
+	}
 	j.mu.RLock()
 	defer j.mu.RUnlock()
 	gutil.Dump(*j.p)