dix: make DeviceEnterLeaveEvent() anaylzer-friendly

The analyzer giving a false alarm on potential NULL-pointer deref here.
Even though that case can't happen, it's also not immediately clear
to the human reader.

To make both the analyzer as well human reader happier, reformulating
this function to by using fast-return pattern.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

dix/events.c

@@ -4747,7 +4747,6 @@ DeviceEnterLeaveEvent(DeviceIntPtr mouse,
                       int mode, int detail, WindowPtr pWin, Window child)
 {
     GrabPtr grab = mouse->deviceGrab.grab;
-    xXIEnterEvent *event;
     WindowPtr focus;
     int filter;
     int btlen, len, i;
@@ -4761,7 +4760,10 @@ DeviceEnterLeaveEvent(DeviceIntPtr mouse,
     btlen = bytes_to_int32(btlen);
     len = sizeof(xXIEnterEvent) + btlen * 4;
 
-    event = calloc(1, len);
+    xXIEnterEvent *event = calloc(1, len);
+    if (!event)
+        return;
+
     event->type = GenericEvent;
     event->extension = IReqCode;
     event->evtype = type;