efi_loader: fix efi_get_next_variable_name_mem()

The VariableNameSize parameter is in bytes but u16_strnlen() counts u16.

Fix the parameter check for null termination.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>