mem-cache: Fix panic in Indirect Memory prefetcher

mem-cache: Fix panic in Indirect Memory prefetcher

Memory requests with a size non-power-of-two and less than 8 values were causing
a panic, but there these should be allowed and ignored by the prefetcher.

Change-Id: I86baa60058cc8a7f232d6ba5748d4c24a463c840
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17733
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>