efi_loader: fix efi_convert_device_path_to_text()

Ensure that the string we convert to UTF-16 is NUL terminated even
if the device path only contains end nodes.

Fixes: bd3d75bb ("efi_loader: multi part device paths to text")
Addresses-Coverity: 350434 ("Uninitialized scalar variable")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>