SegFault when append two scores that don't contain any actual Measures
If joining album where first two scores contain no actual Measure objects (e.g. only contains non-measure MeasureBase objects like vbox-only.mscx), then mscore will crash.
The segfault happens in Score::appendScore when de-referencing a null Measure* m at line "Segment* seg = m->getSegment(Segment::Type::KeySig, tickLen)" which was null because tick2measure(tickLen) returns null if can't find a measure. I will submit a quick fix which will only adjust key signatures if that measure exists.
Before someone says this isn't a bug, I will say it is because a score without measures is a valid score that mscore will save and load, and which the album feature will accept as input. Also, I can suggest one plausible use case example: a composer has a title page (which is a .mscz containing just a frame and and image) and a bio page (which is a .mscz of a page of text which is reused and always attached at the beginning of any albums created).
Note: if use completely empty score without any MeasureBase objects (like attached empty.mscx), then mscore will appropriately refuse and popup an error when appending empty.mscx to empty.mscx, or when appending vbox-only.mscx to empty.mscx, although will crash if try append empty.mscx to vbox-only.mscx.