misc: Fixed null-pointer arithmetic error

misc: Fixed null-pointer arithmetic error

Doing arithmetic on a null pointer is undefined behavior in C/C++. Clang
compilers complain when this occurs. As this MACRO is used twice, and
does nothing important, it has been removed in favor of a more simple
solution. A comment has been added explaining the MACRO's removal.

Change-Id: I42d9356179ee0fa5cb20f827af34bb11780ad1a9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/29534
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>