vendor/ldns: upgrade from 1.7.1 to 1.8.3

vendor/ldns: upgrade from 1.7.1 to 1.8.3

Notable changes:

 * bugfix #50: heap Out-of-bound Read vulnerability in rr_frm_str_internal
   reported by pokerfacett.
 * bugfix #51: Heap Out-of-bound Read vulnerability in ldns_nsec3_salt_data
   reported by pokerfacett.
 * Fix memory leak in examples/ldns-testns handle_tcp routine.
 * Detect fixed time memory compare for openssl 0.9.8.
 * Fix -U flag with ldns-signzone. Thanks Ulrich and Jonathan
 * bugfix #166: Grow buffer when writing chars and fixed size
   strings when converting to presentation format, preventing
   potential assersion errors.
 * bugfix #183: Assertion failure with OPT record without rdata.
   This caused packet creation with only a DO bit (for DNSSEC OK)
   to crash. Thanks Anand Buddhdev and others for reporting this
   so quickly.

For details see commit history or changelog on github:

https://github.com/NLnetLabs/ldns/blob/release-1.8.3/Changelog