stub: allocate and zero enough space in legacy x86 handover protocol

stub: allocate and zero enough space in legacy x86 handover protocol

A PE image's memory footprint might be larger than its file size due
to uninitialized memory sections. Normally all PE headers should be
parsed to check the actual required size, but the legacy EFI handover
protocol is only used for x86 Linux bzImages, so we know only the last
section will require extra memory. Use SizeOfImage from the PE header
and if it is larger than the file size, allocate and zero extra memory
before using it.

Fixes https://github.com/systemd/systemd/issues/33816