Assorted fixes for code paths that are not careful with boundary and
error conditions.
* js/coverity-fixes:
test-read-midx: check midx_fill_entry() result
oss-fuzz: handle reftable iterator initialization failures
t/unit-tests: check reftable iterator initialization
rerere: do not record failed conflict resolution data
midx: validate incremental MIDX pack IDs
gpg-interface: make signature-prefix matching length-aware
wrapper: guard writev_in_full() against signed overflow