NetLabel: better error handling involving mls_export_cat()

Upon inspection it looked like the error handling for mls_export_cat() was
rather poor.  This patch addresses this by NULL'ing out kfree()'d pointers
before returning and checking the return value of the function everywhere
it is called.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>
diff --git a/security/selinux/ss/mls.c b/security/selinux/ss/mls.c
index c713af2..2cca8e2 100644
--- a/security/selinux/ss/mls.c
+++ b/security/selinux/ss/mls.c
@@ -640,8 +640,13 @@
 {
 	int rc = -EPERM;
 
-	if (!selinux_mls_enabled)
+	if (!selinux_mls_enabled) {
+		*low = NULL;
+		*low_len = 0;
+		*high = NULL;
+		*high_len = 0;
 		return 0;
+	}
 
 	if (low != NULL) {
 		rc = ebitmap_export(&context->range.level[0].cat,
@@ -661,10 +666,16 @@
 	return 0;
 
 export_cat_failure:
-	if (low != NULL)
+	if (low != NULL) {
 		kfree(*low);
-	if (high != NULL)
+		*low = NULL;
+		*low_len = 0;
+	}
+	if (high != NULL) {
 		kfree(*high);
+		*high = NULL;
+		*high_len = 0;
+	}
 	return rc;
 }