cpuCompare: Fix crash on unexpected CPU XML

When comparing a CPU without <model> element, such as

    <cpu>
        <topology sockets='1' cores='1' threads='1'/>
    </cpu>

libvirt would happily crash without warning.

src/cpu/cpu.c

@@ -90,6 +90,12 @@ cpuCompareXML(virCPUDefPtr host,
     if (cpu == NULL)
         goto cleanup;
 
+    if (!cpu->model) {
+        virCPUReportError(VIR_ERR_OPERATION_INVALID,
+                "%s", _("no CPU model specified"));
+        goto cleanup;
+    }
+
     ret = cpuCompare(host, cpu);
 
 cleanup: