efi_loader: Check for a valid fw_name before auto generating GUIDs

efi_loader: Check for a valid fw_name before auto generating GUIDs

The gen_v5_guid() is a void and does no error checking with pointers
being available etc. Instead it expects all things to be in place to
generate GUIDs. If a board capsule definition is buggy and does not
define the firmware names when enabling capsule updates, the board will
crash trying to bring up the EFI subsystem.

Check for a valid firmware name before generating GUIDs.

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>