Fix off-by-one error in determining the sync committee when POSTing sync committee messages through the REST API

Author: zah

beacon_chain/validators/validator_duties.nim

@@ -268,7 +268,7 @@ proc sendSyncCommitteeMessages*(node: BeaconNode,
 
           for index, msg in msgs.pairs():
             if msg.validator_index < lenu64(state.data.validators):
-              let msgPeriod = sync_committee_period(msg.slot)
+              let msgPeriod = sync_committee_period(msg.slot + 1)
               if msgPeriod == curPeriod:
                 resCur[msg.validator_index] = index
               elif msgPeriod == nextPeriod: