fixed interface usage of custom auth check function

src/shellmatta_auth.c

@@ -116,7 +116,7 @@ void checkPassword(shellmatta_handle_t handle, uint32_t userId, char *password)
 
     if (NULL != inst->checkFct)
     {
-        approvedUserId = inst->checkFct(userId, password);
+        approvedUserId = inst->checkFct(userId, password) == SHELLMATTA_OK ? userId : 0u;
     }
     else
     {