Fix getting return address in elf/tst-audit28.c.

Starting with commit 1bcfe0f7, the
test was enhanced and the object for __builtin_return_address (0)
is searched with _dl_find_object.

Unfortunately on e.g. s390 (31bit), a postprocessing step is needed
as the highest bit has to be masked out.  This can be done with
__builtin_extract_return_addr.

Without this postprocessing, _dl_find_object returns with -1 and the
content of dlfo is invalid, which may lead to segfaults in basename.
Therefore those checks are now only done on success.
Reviewed-by: Florian Weimer <fweimer@redhat.com>