cmd/compile/internal/riscv: Fix address generation for Needzero

Was previously not zeroing the correct address range, leading to garbage
in local variables, leading to crashes in select statement execution.

Change-Id: I10e985b841d09a015a948ade204dcbca8965e800
Reviewed-on: https://review.gerrithub.io/352837


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>