qt56: fix CVE-2018-{15518,19873,19870,19871,19865,19869}

 * CVE-2018-15518, Qt Base: “double free or corruption” in QXmlStreamReader
 * CVE-2018-19873, Qt Base: QBmpHandler segfault on malformed BMP file
 * CVE-2018-19870, Qt Base: Check for QImage allocation failure in qgifhandler
 * CVE-2018-19871, Qt Imageformats: QImage: QTgaFile CPU exhaustion
 * CVE-2018-19865, Qt Virtual Keyboard: Qt Virtual Keyboard logs all key presses
 * CVE-2018-19869, Qt Svg: Fix crash when parsing malformed url reference

More details can be obtained from the Qt annoucement [1].

[1] https://blog.qt.io/blog/2018/12/04/qt-5-11-3-released-important-security-updates/