efi_loader: fix handling of DHCP acknowledge

The dhcp command may be executed after the first UEFI command.
We should still update the EFI_PXE_BASE_CODE_PROTOCOL.

Don't leak content of prior acknowledge packages.

Handle failing allocation when calling malloc().

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