Bug 1665402

If you try editing text in a text frame and the text is so long that it spans over several lines, Inkscape crashes whenever you hit the END key and then the DOWN ARROW key. You have to this type of movement when you double click a text frame to start editing the text and the cursor is put in the middle of the text (let's say on the second line of four). You then press the END key to arrive at the end of the text which does not happen, because you only arrive at the end of that respective line. They you hit the DOWN ARROW key to come to the next line and Inkscape crashes in that moment.

To reproduce:

1. Start Inkscape.
2. Add a text frame and start writing a text.
3. Make it long so that expands over several lines.
4. Click in the middle of the text to put the cursor on a non-ultimate line.
5. Hit END key to move to the end of the text (that does not happen, because you arrive at the end of that line only.
6. Press DOWN ARROW to move to the next line.
7. Inkscape crashes.

It leaves the following trace:

/usr/include/c++/8/bits/stl_vector.h:950: std::vector<_Tp, _Alloc>::const_reference std::vector<_Tp, _Alloc>::operator[](std::vector<_Tp, _Alloc>::size_type) const [with _Tp = Inkscape::Text::Layout::Character; _Alloc = std::allocator<Inkscape::Text::Layout::Character>; std::vector<_Tp, _Alloc>::const_reference = const Inkscape::Text::Layout::Character&; std::vector<_Tp, _Alloc>::size_type = long unsigned int]: Assertion '__builtin_expect(__n < this->size(), true)' failed.

Software versions:
Fedora 29
Inkscape 0.92 (inkscape-0.92.3-5.fc29.x86_64)