sequencesync: replace Assert(!isnull) on remote data with runtime errors

[NikolayS]

fetch_sequence_data() in src/backend/replication/logical/sequencesync.c asserts remote-returned columns are non-NULL (~lines 285–338). A publisher returning NULL (version skew, pooler, malicious server) crashes assert builds and feeds DatumGetInt64(0) garbage in production. Convert to ereport(ERROR) following the precedent of 3cf5264 in libpqwalreceiver.c.