sss_idmap-tests: Fix segmentation fault
    
    I can reproduce it only with clang.
    But it's tipical off by one error.
    
    sh$ ./sss_idmap-tests
    Running suite(s): IDMAP
    Segmentation fault (core dumped)
    
    Running suite(s): IDMAP
    ==2644== Process terminating with default action of signal 11 (SIGSEGV)
    ==2644==  Access not within mapped region at address 0xA08F430
    ==2644==    at 0x4C2CC53: strcmp (vg_replace_strmem.c:842)
    ==2644==    by 0x4060DA: idmap_test_sid2uid_additional_secondary_slices (sss_idmap-tests.c:451)
    ==2644==    by 0x503C78A: ??? (in /usr/lib64/libcheck.so.0.0.0)
    ==2644==    by 0x503CB7C: srunner_run (in /usr/lib64/libcheck.so.0.0.0)
    ==2644==    by 0x4061EE: main (sss_idmap-tests.c:965)
    ==2644==  If you believe this happened as a result of a stack
    ==2644==  overflow in your program's main thread (unlikely but
    ==2644==  possible), you can try to increase the size of the
    ==2644==  main thread stack using the --main-stacksize= flag.
    ==2644==  The main thread stack size used in this run was 8388608.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    (cherry picked from commit 4f3a996561445ba82c854bb2b674f975f596e884)