plugins: Assert mmu_idx in range before use in qemu_plugin_get_hwaddr

plugins: Assert mmu_idx in range before use in qemu_plugin_get_hwaddr

Coverity reports out-of-bound accesses here.  This should be a
false positive due to how the index is decoded from MemOpIdx.

Fixes: Coverity CID 1487201
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Damien Hedde <damien.hedde@greensocs.com>
Message-Id: <20220401190233.329360-1-richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220929114231.583801-37-alex.bennee@linaro.org>