vtfontcvt: improve .bdf validation

Previously if we had a BBX entry that had invalid values (e.g. bounding
box outside of font bounding box) and failed sscanf (e.g., because it
had fewer than four values) we skipped the BBX value validation and then
triggered an assertion failure.

Reported by: afl
MFC with: r349100
Event: Berlin Devsummit 2019
Sponsored by: The FreeBSD Foundation