Replay r349333 by emaste accidentally reverted by r349352

vtfontcvt: improve .bdf validation

Previously if we had a FONTBOUNDINGBOX or DWIDTH entry that had missing
or invalid values and and failed sscanf, we would proceeded with
partially initialized bounding box / device width variables.

Reported by: afl (FONTBOUNDINGBOX)
MFC with: r349100
Sponsored by: The FreeBSD Foundation