libxl: Resolve possible NULL dereference

If we reached cleanup: prior to allocating cpus, it was possible that
'nr_nodes' had a value, but cpus was NULL leading to a possible NULL
deref. Add a 'cpus' as an end condition to for loop

src/libxl/libxl_conf.c

@@ -196,7 +196,7 @@ libxlCapsInitNuma(libxl_ctx *ctx, virCapsPtr caps)
 
  cleanup:
     if (ret != 0) {
-        for (i = 0; i < nr_nodes; i++)
+        for (i = 0; cpus && i < nr_nodes; i++)
             VIR_FREE(cpus[i]);
         virCapabilitiesFreeNUMAInfo(caps);
     }