zfs_allow_log_destroy parameter NULL

zfs_allow_log_destroy parameter NULL pointer dereference

issues: please see  #4872
Observed during Linux 2.6.32.41 automated testing while running the ZFS Test Suite. Cause ZFS software to produce coredump.

Cause analysis：
In zfs_ioc_log_history function, the implementation of tsd_set function, will he_value of the TSD module is set to null,
resulting in TSD module remove a entry, so he_value of the entry is null,
casue to implement zfs_allow_log_key private function zfs_allow_log_destroy.
zfs_allow_log_destroy parameter is null, the strfree a null. Produce coredump.

Solution：
1, in order to safety,
in the zfs_ioc_log_history function,from the TSD module to get to the poolName,
it is possible for the NULL, so whether the processing of NULL.
if poolname is NULL,return error.

2, zfs_allow_log_key of the private function zfs_allow_log_destroy in the Senate,
   it is possible for the emergence of NULL,
   so for arg release when the judge for the NULL and then strfree it.

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 +6299,8 @@ static void
 zfs_allow_log_destroy(void *arg)
 {
 	char *poolname = arg;
-	strfree(poolname);
+	if (poolname != NULL)
+	    strfree(poolname);
 }
 
 #ifdef DEBUG