Fix for CONC-568:

If multiple threads attempt to connect to a server using a
dynamically loaded authentication plugin the error
"plugin is already loaded" might occur. This is caused
by a race condition if one thread waits for a lock to
load the plugin, while another process which obtained the lock
already loaded the plugin.

The API function mysql_load_plugin_v() now returns
the plugin handle (instead of raising an error and returning
a NULL handle) even if the plugin was already loaded.

libmariadb/mariadb_lib.c

@@ -4060,7 +4060,7 @@ my_bool mariadb_get_infov(MYSQL *mysql, enum mariadb_value value, void *arg, ...
       *((char **)arg)= (char *)ma_pvio_tls_cipher(mysql->net.pvio->ctls);
     else
     #endif
-      goto error;
+      *((char **)arg)= NULL;
     break;
   case MARIADB_CLIENT_ERRORS:
     *((char ***)arg)= (char **)client_errors;