efi_loader: avoid buffer overrun in efi_var_restore()

efi_loader: avoid buffer overrun in efi_var_restore()

The value of buf->length comes from outside U-Boot and may be incorrect.
We must avoid to overrun our internal buffer for excessive values.

If buf->length is shorter than the variable file header, the variable
file is invalid.

Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>