assignment-pattern doesn't work in yosys

[wtatarski]

Log from CI: https://github.com/antmicro/yosys/runs/1790510504?check_suite_focus=true

Segmentation fault (core dumped)
Makefile:44: recipe for target 'uhdm/yosys/test-ast' failed
make: *** [uhdm/yosys/test-ast] Error 139
Error: Process completed with exit code 2.