powerpc: Skip stack validation checking alternate stacks if they are not allocated

Stack validation in early boot can just bail out of checking alternate
stacks if they are not validated yet. Checking against a NULL stack
could cause NULLish pointer values to be considered valid.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20221216115930.2667772-5-npiggin@gmail.com