securityselinuxlabeltest: Prefer virGetLastErrorMessage() over virGetLastError

At the beginning of the test, some preparation work is done. For
instance new virSecurityManager is created. If this fails for
whatever reason, we try to fetch the latest error and print the
error message contained in it. However, if there's a bug in our
code and no error is reported, this approach will lead to crash,
while with virGetLastErrorMessage() it won't.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

tests/securityselinuxlabeltest.c

@@ -354,9 +354,8 @@ mymain(void)
     if (!(mgr = virSecurityManagerNew("selinux", "QEMU",
                                       VIR_SECURITY_MANAGER_DEFAULT_CONFINED |
                                       VIR_SECURITY_MANAGER_PRIVILEGED))) {
-        virErrorPtr err = virGetLastError();
         VIR_TEST_VERBOSE("Unable to initialize security driver: %s\n",
-                err->message);
+                         virGetLastErrorMessage());
         return EXIT_FAILURE;
     }