Don't pass an empty QPrinter to QPrintDialog.

This was garbage collected on Linux when the dialog was opened, leading
to segfaults when the printer was changed in the dialog.

But actually we don't need it at all, so we just drop it.

TODO

@@ -11,9 +11,6 @@ Before 0.1
 Crashes
 =======
 
-- Chosing printer in :print gives us a segfault on Linux, but :printpreview
-  works...
-
 - When following a hint:
 
   QNetworkReplyImplPrivate::error: Internal problem, this method must only be called once.

qutebrowser/browser/curcommand.py

@@ -26,7 +26,7 @@ from functools import partial
 from PyQt5.QtWidgets import QApplication
 from PyQt5.QtCore import pyqtSlot, Qt, QObject, QProcess
 from PyQt5.QtGui import QClipboard
-from PyQt5.QtPrintSupport import QPrinter, QPrintDialog, QPrintPreviewDialog
+from PyQt5.QtPrintSupport import QPrintDialog, QPrintPreviewDialog
 
 import qutebrowser.utils.url as urlutils
 import qutebrowser.utils.message as message
@@ -180,8 +180,7 @@ class CurCommandDispatcher(QObject):
         # If this isn't fixed in Qt 5.3, bug should be reopened.
         tab = self._tabs.cntwidget(count)
         if tab is not None:
-            printer = QPrinter()
+            printdiag = QPrintDialog(tab)
-            printdiag = QPrintDialog(printer, tab)
             printdiag.open(lambda: tab.print(printdiag.printer()))
 
     @cmdutils.register(instance='mainwindow.tabs.cur')