qemu: json: Fix daemon crash on handling domain shutdown event

commit a8eba503 added further checking of the guest shutdown cause, but
this enhancement is available since qemu 2.10, causing a crash because
of a NULL pointer dereference on older qemus.

Thread 1 "libvirtd" received signal SIGSEGV, Segmentation fault.
0x00007ffff72441af in virJSONValueObjectGet (object=0x0,
                                             key=0x7fffd5ef11bf "guest")
    at util/virjson.c:769
769	    if (object->type != VIR_JSON_TYPE_OBJECT)
(gdb) bt
0  in virJSONValueObjectGet
1  in virJSONValueObjectGetBoolean
2  in qemuMonitorJSONHandleShutdown
3  in qemuMonitorJSONIOProcessEvent
4  in qemuMonitorJSONIOProcessLine
5  in qemuMonitorJSONIOProcess
6  in qemuMonitorIOProcess
Signed-off-by: NErik Skultety <eskultet@redhat.com>

src/qemu/qemu_monitor_json.c

@@ -528,7 +528,7 @@ static void qemuMonitorJSONHandleShutdown(qemuMonitorPtr mon, virJSONValuePtr da
     bool guest = false;
     virTristateBool guest_initiated = VIR_TRISTATE_BOOL_ABSENT;
 
-    if (virJSONValueObjectGetBoolean(data, "guest", &guest) == 0)
+    if (data && virJSONValueObjectGetBoolean(data, "guest", &guest) == 0)
         guest_initiated = guest ? VIR_TRISTATE_BOOL_YES : VIR_TRISTATE_BOOL_NO;
 
     qemuMonitorEmitShutdown(mon, guest_initiated);