efi_loader: fix pe reloc pointer overrun

efi_loader: fix pe reloc pointer overrun

The fix provided by 997fc12ec91 is actually introducing
a buffer overrun, and the overrun is effective if the
memory after the reloc section is not zeroed.
Probably that's why this bug is not always noticeable.

The problem is that 8-bytes 'rel' pointer can be 4-bytes aligned
according to the PE Format, so the actual relocate function can
take values after the reloc section.

One example is the following dump from the reloc section:

    bce26000: 3000 0000 000c 0000 0000 0000 0000 0000
    bce26010: 7c00 9340 67e0 f900 1c00 0ea1 a400 0f20

This section has two relocations at offset bce26008 and bce2600a,
however the given size (rel_size) for this relocation is 16-bytes
and this is coming form the efi image Misc.VirtualSize, so in this
case the 'reloc' pointer ends at affset bce2600c and is taken as
valid and this is where the overflow is.

In our system we see this problem when we are starting the
Boot Guard efi image.

This patch is fixing the overrun while preserving the fix done
by 997fc12ec91.

Signed-off-by: Aleksandar Gerasimovski <aleksandar.gerasimovski@belden.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>