API: Fix null pointer in attempt to Sudo if not logged in (#5872) (#5884)

Backport of #5872 to v1.7

Signed-off-by: Andrew Thornton <art27@cantab.net>

routers/api/v1/api.go

@@ -85,7 +85,7 @@ func sudo() macaron.Handler {
 		}
 
 		if len(sudo) > 0 {
-			if ctx.User.IsAdmin {
+			if ctx.IsSigned && ctx.User.IsAdmin {
 				user, err := models.GetUserByName(sudo)
 				if err != nil {
 					if models.IsErrUserNotExist(err) {