Fix NULL deref in kcf_remove_mech_provider

In the default case the function must return to avoid dereferencing
'prov_mech' which will be NULL.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: candychencan <chen.can2@zte.com.cn>
Closes #5134

module/icp/core/kcf_mech_tabs.c

@@ -659,7 +659,9 @@ kcf_remove_mech_provider(char *mech_name, kcf_provider_desc_t *prov_desc)
 		mech_entry->me_sw_prov = NULL;
 		break;
 	default:
-		break;
+		/* unexpected crypto_provider_type_t */
+		mutex_exit(&mech_entry->me_mutex);
+		return;
 	}
 
 	mutex_exit(&mech_entry->me_mutex);