sbuf: fix WriteBuf panic on zero-length vector

db/ztests/ls.yaml

@@ -1,6 +1,8 @@
 script: |
   export SUPER_DB=test
   super db init -q
+  super db ls -s
+  echo ===
   super db create -q -orderby ts:desc logs
   super db load -q -use logs babble.sup
   super db ls -f bsup | super -S -c "drop id,ts" -
@@ -17,6 +19,7 @@ inputs:
 outputs:
   - name: stdout
     data: |
+      ===
       type "order.Which" = string
       type "field.Path" = [string]
       type "order.SortKey" = {

sbuf/materialize.go

@@ -115,7 +115,9 @@ func (d *Dematerializer) parentPull(done bool) (Batch, error) {
 }
 
 func WriteVec(w sio.Writer, vec vector.Any) error {
-	for _, val := range Materialize(vec).Values() {
+	var b scode.Builder
+	for i := range vec.Len() {
+		val := vector.ValueAt(&b, vec, i)
 		if err := w.Write(val); err != nil {
 			return err
 		}

service/ztests/ls-pools.yaml

@@ -1,5 +1,7 @@
 script: |
   source service.sh
+  super db ls -s
+  echo ===
   super db create p1
   super db create p2
   echo ===
@@ -21,6 +23,7 @@ inputs:
 outputs:
   - name: stdout
     regexp: |
+      ===
       table created: p1 [0-9a-f-]{36}
       table created: p2 [0-9a-f-]{36}
       ===