Fix crash.
Fix crash.

addFilter/removeFilter with 0 as the argument causes a crash.
warn and return instead.

enthought/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,enthought/qt-mobility,kaltsi/qt-mobility,KDE/android-qt-mobility,kaltsi/qt-mobility,KDE/android-qt-mobility,qtproject/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,tmcguire/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,qtproject/qt-mobility