qemu: Fix possible crash in qemuProcessSetVcpuAffinities

In case when <vcpu ... cpuset=""> is not specified, the vcpupin array is
not guaranteed to be allocated to def->vcpus. This would cause a crash
for TCG since it does not report thread IDs for vCPUs.

src/qemu/qemu_process.c

@@ -2376,7 +2376,7 @@ qemuProcessSetVcpuAffinities(virDomainObjPtr vm)
         /* If any CPU has custom affinity that differs from the
          * VM default affinity, we must reject it
          */
-        for (n = 0; n < def->vcpus; n++) {
+        for (n = 0; n < def->cputune.nvcpupin; n++) {
             if (!virBitmapEqual(def->cpumask,
                                 def->cputune.vcpupin[n]->cpumask)) {
                 virReportError(VIR_ERR_OPERATION_INVALID,