master: Crash on selecting certain fonts

Steps to reproduce:

• open Inkscape
• use testfont.ttf (I used arrow keys in the font list in the text tool's tool controls)
• restart Inkscape
• use Decovar variable font (https://github.com/TypeNetwork/Decovar) (demo font!, OFL-licensed) DecovarAlpha-VF.ttf

What happened?

• crash, as soon as the font SVGFont or Decovar is reached : inkscape_debug_trace_2019.09.11.03.12.15.txt inkscape_debug_trace_2019.09.11.03.33.18.txt

What should have happened?

• text should have shown in the selected font (works in 0.92.x and in LibreOffice. Decovar definitely worked when I made the screenshots for the 1.0 release notes.)

Inkscape Version and Operating System:

• Inkscape Version: 1.0beta0 (1d85f8ed2a, 2019-09-11)
• Operating System: Linux Mint 19.2