Kate and kwrite both segfault on clicking the "code folding markers" area with syntax highlighting enabled - subject to certain other, seemingly arbitrary, conditions, which I have documented in the 'steps to reproduce' section

Reproducible: Always

Steps to Reproduce:
1. Open new instance of kwrite or kate.
2. Ensure that the "code folding markers" area is shown.
3. Change syntax highlighting to anything but 'None', e.g. 'C++'.
4. Click on the grey folding-marker area anywhere *below the last line* of text (i.e. the empty first line).

Actual Results:  
Crash! Segfault!

Expected Results:  
Nothing should happen.

- Segfault occurs in libKF5TextEditor.so.5.13.0, at KateBuffer::computeFoldingRangeForStartLine(int).
- This is probably the same crash as in bug 339894 but with different causes.

Some other notes:
- Step 2 is not optional; it won't crash without syntax highlighting enabled.
- The bug disappears after you have managed click the grey area once without a crash.