xcb: check for nullptr when reading AT_SPI_BUS property

We always have to check the return value of xcb_get_property(),
but this code did not do it. These xcb functions do not check
for validity of the pointer, so we have to make sure that we pass-in
something valid:

 void *
 xcb_get_property_value (const xcb_get_property_reply_t *R)
 {
    return (void *) (R + 1);
 }

 int
 xcb_get_property_value_length (const xcb_get_property_reply_t *R)
 {
    return (R->value_len * (R->format / 8));
 }

Fixes: QTBUG-74067
Change-Id: Iabbc81e6079d96c7314d16dd78783de07f9ad629
Reviewed-by: Mikhail Svetkin <mikhail.svetkin@qt.io>
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

src/plugins/platforms/xcb/qxcbnativeinterface.cpp

@@ -412,12 +412,15 @@ void *QXcbNativeInterface::atspiBus()
         auto reply = Q_XCB_REPLY(xcb_get_property, defaultConnection->xcb_connection(),
                                      false, defaultConnection->rootWindow(),
                                      atspiBusAtom, XCB_ATOM_STRING, 0, 128);
-        Q_ASSERT(!reply->bytes_after);
+        if (!reply)
+            return nullptr;
+
         char *data = (char *)xcb_get_property_value(reply.get());
         int length = xcb_get_property_value_length(reply.get());
         return new QByteArray(data, length);
     }
-    return 0;
+
+    return nullptr;
 }
 
 void QXcbNativeInterface::setAppTime(QScreen* screen, xcb_timestamp_t time)