out-of-bounds access on Enter-up-down in new text field

https://bugzilla.redhat.com/show_bug.cgi?id=1612618

Reproduce using:
1. Select text tool (F8)
2. Click on empty canvas
3. Hit Enter
4. Hit Up
5. Hit Down

Backport from 1.0alpha master
Cherry picked from 5ed5cde1

src/libnrtype/Layout-TNG-OutIter.cpp

@@ -788,6 +788,9 @@ bool Layout::iterator::nextLineCursor(int n)
                          - _parent_layout->_chunks[_parent_layout->_spans[_parent_layout->_lineToSpan(line_index)].in_chunk].left_x;
     }
     _char_index = _parent_layout->_cursorXOnLineToIterator(line_index + n, _x_coordinate)._char_index;
+    if (_char_index == _parent_layout->_characters.size())
+        _glyph_index = _parent_layout->_glyphs.size();
+    else
         _glyph_index = _parent_layout->_characters[_char_index].in_glyph;
     return true;
 }