mtd: uboot: Fix hanging during mtd list command

Some boards (like omap3_logic) hang when trying to access
address 0. This happens when executing the new 'mtd list' command.
This patch enhances the checks for conditions that would
preclude mtd_probe_devices() from operating.

Fixes: 5db66b3aee ("cmd: mtd: add 'mtd' command")
Suggested-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>

drivers/mtd/mtd_uboot.c

@@ -104,7 +104,10 @@ int mtd_probe_devices(void)
 	mtd_probe_uclass_mtd_devs();
 
 	/* Check if mtdparts/mtdids changed since last call, otherwise: exit */
-	if (!strcmp(mtdparts, old_mtdparts) && !strcmp(mtdids, old_mtdids))
+	if ((!mtdparts && !old_mtdparts && !mtdids && !old_mtdids) ||
+	    (mtdparts && old_mtdparts && mtdids && old_mtdids &&
+	     !strcmp(mtdparts, old_mtdparts) &&
+	     !strcmp(mtdids, old_mtdids)))
 		return 0;
 
 	/* Update the local copy of mtdparts */
@@ -140,6 +143,10 @@ int mtd_probe_devices(void)
 		}
 	}
 
+	/* If either mtdparts or mtdids is empty, then exit */
+	if (!mtdparts || !mtdids)
+		return 0;
+
 	/* Start the parsing by ignoring the extra 'mtdparts=' prefix, if any */
 	if (strstr(mtdparts, "mtdparts="))
 		mtdparts += 9;