vtfontcvt: Avoid dead store in add_char

[emaste]

Summary

The fallback glyph is stored at index 0, and does not need to be
inserted into a mapping.

Previously there was a dead store of add_glyph's return value for the
fallback case, which upset Clang's static analyzer.  Now, cast the
return value to (void) to make it clear this is intentional.

Also change add_glyph's fallback parameter to a c99 bool to make its use
more clear.

Sponsored by: The FreeBSD Foundation