mtdparts: fix write through NULL pointer

The "mtdparts add" command wrote through a NULL pointer - on many
systems this went unnoticed (PowerPC has writable RAM there, some ARM
systems have ROM where a write has no effect), but on arm1136
(i.MX31) it crashed the system.

Add appropriate checks.
Signed-off-by: NWolfgang Denk <wd@denx.de>

common/cmd_mtdparts.c

@@ -837,14 +837,16 @@ static int device_parse(const char *const mtd_dev, const char **ret, struct mtd_
 	u32 offset;
 	int err = 1;
 
-	p = mtd_dev;
+	DEBUGF("===device_parse===\n");
+
+	assert(retdev);
 	*retdev = NULL;
-	*ret = NULL;
 
-	DEBUGF("===device_parse===\n");
+	if (ret)
+		*ret = NULL;
 
 	/* fetch <mtd-id> */
-	mtd_id = p;
+	mtd_id = p = mtd_dev;
 	if (!(p = strchr(mtd_id, ':'))) {
 		printf("no <mtd-id> identifier\n");
 		return 1;
@@ -913,11 +915,14 @@ static int device_parse(const char *const mtd_dev, const char **ret, struct mtd_
 	/* check for next device presence */
 	if (p) {
 		if (*p == ';') {
+			if (ret)
 				*ret = ++p;
 		} else if (*p == '\0') {
+			if (ret)
 				*ret = p;
 		} else {
 			printf("unexpected character '%c' at the end of device\n", *p);
+			if (ret)
 				*ret = NULL;
 			return 1;
 		}