drm: Do not call the egl backend pointer when it's null
It can be null, that's why we are also checking it down in the same function.
#0 0x00007f94c4959a42 in KWin::DrmPipeline::checkTestBuffer()::$_3::operator()(QSharedPointer<KWin::DrmBuffer> const&) const (this=0x7ffe63df4888, buf=...)
at /home/apol/devel/frameworks/kwin/src/plugins/platforms/drm/drm_pipeline.cpp:278
#1 0x00007f94c49571a1 in KWin::DrmPipeline::checkTestBuffer() (this=0x564c2120ca50) at /home/apol/devel/frameworks/kwin/src/plugins/platforms/drm/drm_pipeline.cpp:287
#2 0x00007f94c4957777 in KWin::DrmPipeline::commitPipelines(QVector<KWin::DrmPipeline*> const&, KWin::DrmPipeline::CommitMode) (pipelines=..., mode=KWin::DrmPipeline::CommitMode::Commit)
at /home/apol/devel/frameworks/kwin/src/plugins/platforms/drm/drm_pipeline.cpp:140
#3 0x00007f94c4958324 in KWin::DrmPipeline::atomicCommit() (this=0x564c2120ca50) at /home/apol/devel/frameworks/kwin/src/plugins/platforms/drm/drm_pipeline.cpp:111
#4 0x00007f94c49580c1 in KWin::DrmPipeline::present(QSharedPointer<KWin::DrmBuffer> const&) (this=0x564c2120ca50, buffer=...) at /home/apol/devel/frameworks/kwin/src/plugins/platforms/drm/drm_pipeline.cpp:91
#5 0x00007f94c49379e5 in KWin::DrmOutput::present(QSharedPointer<KWin::DrmBuffer> const&, QRegion) (this=0x564c2120c210, buffer=..., damagedRegion=...) at /home/apol/devel/frameworks/kwin/src/plugins/platforms/drm/drm_output.cpp:395
#6 0x00007f94c493c32e in KWin::DrmQPainterBackend::endFrame(KWin::AbstractOutput*, QRegion const&) (this=0x564c211a12f0, output=0x564c2120c210, damage=...)