[Race] Fix datarace in GetLevel

`std.Level` is protected by mutex in setter (SetLevel),
so it must be protected in geetter (GetLevel) too.

Signed-off-by: Anton Tiurin <noxiouz@yandex.ru>

exported.go

diff --git a/exported.go b/exported.go
index fd092fc..a67e1b8 100644
--- a/exported.go
+++ b/exported.go
@@ -36,6 +36,8 @@
 
 // GetLevel returns the standard logger level.
 func GetLevel() Level {
+	std.mu.Lock()
+	defer std.mu.Unlock()
 	return std.Level
 }
 

logrus_test.go

diff --git a/logrus_test.go b/logrus_test.go
index 7f52c6f..d85dba4 100644
--- a/logrus_test.go
+++ b/logrus_test.go
@@ -5,6 +5,7 @@
 	"encoding/json"
 	"strconv"
 	"strings"
+	"sync"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -281,3 +282,20 @@
 	l, err = ParseLevel("invalid")
 	assert.Equal(t, "not a valid logrus Level: \"invalid\"", err.Error())
 }
+
+func TestGetSetLevelRace(t *testing.T) {
+	wg := sync.WaitGroup{}
+	for i := 0; i < 100; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			if i%2 == 0 {
+				SetLevel(InfoLevel)
+			} else {
+				GetLevel()
+			}
+		}(i)
+
+	}
+	wg.Wait()
+}