mem-cache: Initialize CacheBlk data pointer

mem-cache: Initialize CacheBlk data pointer

Initialize CacheBlk's data pointer as a nullptr.

Change-Id: Ice85b4b11495cad4b0a160ccb9efe1be673e57e2
Reviewed-on: https://gem5-review.googlesource.com/11097
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>