s390/test_unwind: unify error handling paths

Handle the case of "unwind state reliable but addr is 0" like other error
cases in this function and trigger output of failing stacktrace to aid
debugging.

Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>