Fix check for VCF record size

The check for excessive record size in vcf_parse_format() only
looked at individual fields.  It was therefore possible to
exceed the limit and overflow fmt_aux_t::offset by having
multiple fields with a combined size that went over INT_MAX.
Fix by including the amount of memory used so far in the check.

Credit to OSS-Fuzz
Fixes oss-fuzz 24097