xtensa: ISS: don't use string pointer before NULL check

Move strlen call inside the if block that checks string pointer for NULL.
While at it also fix the following coccicheck warning:

./arch/xtensa/platforms/iss/console.c:204:10-11: WARNING comparing
pointer to 0.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>