spapr: fix potential memory leak in spapr_core_plug()

Since commit 5c1da812 ("spapr: Remove unnecessary differences between
hotplug and coldplug paths"), the CPU DT for the DRC is always allocated.
This causes a memory leak for pseries-2.6 and older machine types, that
don't support CPU hotplug and don't allocate DRCs for CPUs.
Reported-by: NBharata B Rao <bharata@linux.vnet.ibm.com>
Signed-off-by: NGreg Kurz <groug@kaod.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr.c

@@ -2974,8 +2974,6 @@ static void spapr_core_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
     CPUState *cs = CPU(core->threads);
     sPAPRDRConnector *drc;
     Error *local_err = NULL;
-    void *fdt = NULL;
-    int fdt_offset = 0;
     int smt = kvmppc_smt_threads();
     CPUArchId *core_slot;
     int index;
@@ -2991,9 +2989,12 @@ static void spapr_core_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
 
     g_assert(drc || !mc->has_hotpluggable_cpus);
 
-    fdt = spapr_populate_hotplug_cpu_dt(cs, &fdt_offset, spapr);
-
     if (drc) {
+        void *fdt;
+        int fdt_offset;
+
+        fdt = spapr_populate_hotplug_cpu_dt(cs, &fdt_offset, spapr);
+
         spapr_drc_attach(drc, dev, fdt, fdt_offset, &local_err);
         if (local_err) {
             g_free(fdt);