fontselect: deal with potential ASS_FontProvider.subst_font failure
fontselect: deal with potential ASS_FontProvider.subst_font failure

Don't assume it sets all fields. If it doesn't return any font names at
all, go with the default.

stps/libass,MrSmile/libass,sanyaade-g2g-repos/libass,stps/libass,MrSmile/libass,libass/libass,astiob/libass,stps/libass,torque/libass,grigorig/libass,torque/libass,mwgoldsmith/libass,Akemi/libass,Akemi/libass,sanyaade-g2g-repos/libass,xbmc/libass,Akemi/libass,ShiftMediaProject/libass,torque/libass,astiob/libass,ShiftMediaProject/libass,mwgoldsmith/libass,xbmc/libass,sanyaade-g2g-repos/libass,grigorig/libass,libass/libass,mwgoldsmith/libass