slw: don't be tricky with pointer math
slw: don't be tricky with pointer math

Mainly because this trips up some static analysis on resource usage
and instead of having someone go back and prove it, add an assert
and keep around a (no doubt optimized out) variable.

Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

legoater/skiboot,legoater/skiboot,legoater/skiboot,stewart-ibm/skiboot,qemu/skiboot,open-power/skiboot,open-power/skiboot,mikey/skiboot,qemu/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,legoater/skiboot,csmart/skiboot,qemu/skiboot,mikey/skiboot,qemu/skiboot,stewart-ibm/skiboot,mikey/skiboot,csmart/skiboot,open-power/skiboot,shenki/skiboot,stewart-ibm/skiboot,shenki/skiboot,shenki/skiboot,csmart/skiboot,open-power/skiboot