recovery: check variable returned by uboot_getenv() is not NULL

The use of this function, which is a wrapper over libuboot_get_env(),
requires checking if the returned string is NULL.
Manipulations of such string without checking whether it's NULL may lead
to segfault errors.
This was seen during firmware update on a device that didn't have the
'dualboot' variable set.

Reported-by: Chandrababu Pigilam <chandrababu.pigilam@digi.com>
Signed-off-by: Hector Palacios <hector.palacios@digi.com>

https://onedigi.atlassian.net/browse/DEL-7645
(cherry picked from commit 8a4484bbd6)

meta-digi-dey/recipes-core/recovery/recovery-utils/recovery-utils/lib/recovery.c

@@ -439,7 +439,7 @@ static int parse_nand_partition_info(char **parts, char **encrypted, unsigned ch
 
 	/* Parse mtdparts for all partition info */
 	ret = uboot_getenv("mtdparts", &var);
-	if (ret) {
+	if (ret || !var) {
 		fprintf(stderr, "Error: getenv 'mtdparts'\n");
 		return ret;
 	}
@@ -978,6 +978,10 @@ int is_dualboot_enabled (void)
 		return 0;
 	}
 
+	/* Consider dualboot not enabled if variable doesn't exist */
+	if (!var)
+		return 0;
+
 	/* Is dualboot enabled */
 	if (!strcmp(var, "no"))
 		ret = 0;