AppServer: check for NULL bitmap.

Consistently check for NULL the ServerBitmap pointer.
There seems to be cases where BView::DragMessage could pass an invalid
Bitmap token to app_server.
Maybe it's when a client only bitmap is passed, I don't know.
Anyway, this is defensive programming, and at least we check for NULL
consistently now.
This fixes #11681.
Note that SuperFreeCell still crashes, but at least app_server doesn't crash.

src/servers/app/EventDispatcher.cpp

@@ -612,7 +612,8 @@ EventDispatcher::SetDragMessage(BMessage& message,
 
 	if (fLastButtons == 0) {
 		// mouse buttons has already been released or was never pressed
-		bitmap->ReleaseReference();
+		if (bitmap != NULL)
+			bitmap->ReleaseReference();
 		return;
 	}
 

src/servers/app/ServerWindow.cpp

@@ -2074,7 +2074,8 @@ fDesktop->UnlockSingleWindow();
 						fDesktop->EventDispatcher().SetDragMessage(dragMessage,
 							bitmap, offset);
 fDesktop->LockSingleWindow();
-						bitmap->ReleaseReference();
+						if (bitmap != NULL)
+							bitmap->ReleaseReference();
 				}
 				delete[] buffer;
 			}