Initialize Vt102Emulation::m_currentImage

There is a very common crash report in ~QImage() called on Vt102Emulation::m_currentImage (implicitly) from ~Vt102Emulation(). This variable is uninitialized unless sixel is used. My guess is that for most (or even all) those reports sixel is not in use.

Initialize this field to a null image, in case this is the problem.

See, for example: https://crash-reports.kde.org/organizations/kde/issues/7607/?project=6&query=release%3Akonsole%4024.02.0&referrer=release-issue-stream https://crash-reports.kde.org/organizations/kde/issues/12067/?project=6&query=&referrer=project-issue-stream