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>
diff --git a/cmd/dm.c b/cmd/dm.c
index 108707c..7a90685 100644
--- a/cmd/dm.c
+++ b/cmd/dm.c
@@ -41,7 +41,7 @@
 }
 
 static int do_dm_dump_drivers(cmd_tbl_t *cmdtp, int flag, int argc,
-			       char * const argv[])
+			      char * const argv[])
 {
 	dm_dump_drivers();
 
@@ -94,5 +94,5 @@
 	"tree          Dump driver model tree ('*' = activated)\n"
 	"dm uclass        Dump list of instances for each uclass\n"
 	"dm devres        Dump list of device resources for each device\n"
-	"dm drivers       Dump list of drivers and their compatible strings\n"
+	"dm drivers       Dump list of drivers and their compatible strings"
 );