efi_loader: catch out of memory in file_open

If calloc() return NULL, don't dereference it.

Fixes: 2a92080d ("efi_loader: add file/filesys support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>