libefivar: Fix a buffer overread.

[jhb]

Summary

DevPathToTextUsbWWID allocates a separate copy of the SerialNumber
string to append a null terminator if the original string is not
null terminated. However, by using AllocateCopyPool, it tries to
copy 'Length + 1' words from the existing string containing 'Length'
characters into the target string. Split the copy out to only
copy 'Length' characters instead.

Reported by: GCC 12 -Wstringop-overread

[imp]

Change looks good to me...

But this is 'upstream' code hiding here. It comes from edk2.

So this needs to be upstreamed to MdePkg/Library/UefiDevicePathLib/DevicePathToText.c

I'll bet that this code has never been called on freeBSD though :)

[emaste]

https://github.com/tianocore/edk2/blob/master/MdePkg/Library/UefiDevicePathLib/DevicePathToText.c#L988

[jhb]

Upstream PR: https://github.com/tianocore/edk2/pull/3437

Their docs claim I need to join a mailings list and mail the patch there. I'll try just doing a PR first as I don't want all that e-mail.