vecstore: when reusing memory, set length to the desired length

Previously, when reusing memory for the output field, PKDecoder.Init()
would truncate the output field instead of setting it to the desired
length. This could lead to a runtime out of bounds memory reference.

Epic: none
Release note (bug fix): An issue that could cause vector index uses to
throw an out of bounds access exception has been fixed.

Author: mw5h

pkg/sql/vecindex/vecstore/store_txn.go

@@ -384,7 +384,7 @@ func (d *PKDecoder) Init(fetchSpec *fetchpb.IndexFetchSpec) {
 	if cap(d.output) < len(fetchSpec.FetchedColumns) {
 		d.output = make(rowenc.EncDatumRow, len(fetchSpec.FetchedColumns))
 	} else {
-		d.output = d.output[:0]
+		d.output = d.output[:len(fetchSpec.FetchedColumns)]
 	}
 	for i, col := range fetchSpec.FetchedColumns {
 		d.colOrdMap.Set(col.ColumnID, i)