tests: libxl: Fix a segfault when libxl configuration setup fails

This commit fixes a segmentation fault caused by missing conditional to
check if libxl configuration was properly created by the test. If the
configuration was not properly created, libxlDriverConfigNew() function
will return NULL and cause a segfault at cfg->caps = NULL during the
cleanup.
Signed-off-by: NJulio Faracco <jcfaracco@gmail.com>
Reviewed-by: NErik Skultety <eskultet@redhat.com>

tests/libxlxml2domconfigtest.c

@@ -138,7 +138,8 @@ testCompareXMLToDomConfig(const char *xmlfile,
     libxl_domain_config_dispose(&actualconfig);
     libxl_domain_config_dispose(&expectconfig);
     xtl_logger_destroy(log);
-    cfg->caps = NULL;
+    if (cfg)
+        cfg->caps = NULL;
     virObjectUnref(cfg);
     return ret;
 }