Xext: return BadAccess if PickPointer fails (#45796)

PickPointer or PickKeyboard return NULL, all MDs are currently disabled and
we cannot emulate a core event. This wasn't anticipated by the protocol, so
we don't really have an error code we may use here - BadAccess is simply the
least bad of the possible ones.

And returning BadAccess beats crashing the server.

X.Org Bug 45796 <http://bugs.freedesktop.org/show_bug.cgi?id=45796>

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>

Xext/xtest.c

@@ -118,6 +118,10 @@ ProcXTestCompareCursor(ClientPtr client)
     rc = dixLookupWindow(&pWin, stuff->window, client, DixGetAttrAccess);
     if (rc != Success)
         return rc;
+
+    if (!ptr)
+        return BadAccess;
+
     if (stuff->cursor == None)
         pCursor = NullCursor;
     else if (stuff->cursor == XTestCurrentCursor)
@@ -307,9 +311,15 @@ ProcXTestFakeInput(ClientPtr client)
             return BadValue;
         }
 
+        /* Technically the protocol doesn't allow for BadAccess here but
+         * this can only happen when all MDs are disabled.  */
+        if (!dev)
+            return BadAccess;
+
         dev = GetXTestDevice(dev);
     }
 
+
     /* If the event has a time set, wait for it to pass */
     if (ev->u.keyButtonPointer.time) {
         TimeStamp activateTime;