KRB: Prevent dereference of a null pointer

Reported by: scan-build
    krb5_free_principal(cc->context, ccprinc);
                        ^~
warning: Access to field 'context' results in a dereference of a null pointer
(loaded from variable 'cc')

Variable 'cc' needn't be initialised if function sss_open_ccache_as_user
fails.

Reviewed-by: Sumit Bose <sbose@redhat.com>

src/providers/krb5/krb5_utils.c

@@ -975,8 +975,10 @@ errno_t sss_krb5_check_ccache_princ(uid_t uid, gid_t gid,
     ret = ERR_NOT_FOUND;
 
 done:
-    krb5_free_principal(cc->context, ccprinc);
-    krb5_free_principal(cc->context, kprinc);
+    if (cc) {
+        krb5_free_principal(cc->context, ccprinc);
+        krb5_free_principal(cc->context, kprinc);
+    }
     talloc_free(tmp_ctx);
     return ret;
 }