Fix zfs_allow_log_destroy() NULL dereference

In zfs_ioc_log_history() function the tsd_set() function is called
with NULL which causes the zfs_allow_log_destroy() to be run.  In
this case the passed value will be NULL.  This is normally entirely
safe because strfree() maps directly to kfree() which may be passed
a NULL.  However, since alternate implementations of strfree() may
not handle this gracefully add a check for NULL.

Observed under an embedded Linux 2.6.32.41 kernel running the
automated testing while running the ZFS Test Suite.

Signed-off-by: caoxuewen <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #4872

module/zfs/zfs_ioctl.c

@@ -3345,6 +3345,8 @@ zfs_ioc_log_history(const char *unused, nvlist_t *innvl, nvlist_t *outnvl)
 	 * we clear the TSD here.
 	 */
 	poolname = tsd_get(zfs_allow_log_key);
+	if (poolname == NULL)
+		return (SET_ERROR(EINVAL));
 	(void) tsd_set(zfs_allow_log_key, NULL);
 	error = spa_open(poolname, &spa, FTAG);
 	strfree(poolname);
@@ -6297,7 +6299,9 @@ static void
 zfs_allow_log_destroy(void *arg)
 {
 	char *poolname = arg;
-	strfree(poolname);
+
+	if (poolname != NULL)
+		strfree(poolname);
 }
 
 #ifdef DEBUG