cmd: Add test and fix bugs for dm drivers

Add a test for the dm drivers command. Also fix a null pointer dereference
revealed by said test.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

test/py/tests/test_dm.py

@@ -0,0 +1,17 @@
+# SPDX-License-Identifier: GPL-2.0
+# Copyright (C) 2020 Sean Anderson
+
+import pytest
+
+@pytest.mark.buildconfigspec('cmd_dm')
+def test_dm_drivers(u_boot_console):
+    """Test that each driver in `dm tree` is also listed in `dm drivers`."""
+    response = u_boot_console.run_command('dm tree')
+    driver_index = response.find('Driver')
+    assert driver_index != -1
+    drivers = (line[driver_index:].split()[0]
+               for line in response[:-1].split('\n')[2:])
+
+    response = u_boot_console.run_command('dm drivers')
+    for driver in drivers:
+        assert driver in response