sandbox: fix sandbox_reset()

state_uninit() and dm_uninit() are mutually exclusive:

state_uninit() prints via drivers. So it cannot be executed after
dm_uninit().

dm_uninit() requires memory. So it cannot be executed after state_uninit()
which releases all memory.

Just skip dm_uninit() when resetting the sandbox. We will wake up in a new
process and allocate new memory. So this cleanup is not required. We don't
do it in sandbox_exit() either.

This avoids a segmentation error when efi_reset_system_boottime() is
invoked by a UEFI application.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>