row: only store the accounted for memory if the reservation is approved

Previously, we would update the counter about the reserved memory before
doing the reservation. If that reservation is denied, then later on, in
`txnKVFetcher.close` we could try to release more memory than we
registered. This is now fixed.

Release note: None

pkg/sql/row/kv_batch_fetcher.go

@@ -356,10 +356,11 @@ func (f *txnKVFetcher) SetupNextFetch(
 
 	// Account for the memory of the spans that we're taking the ownership of.
 	if f.acc != nil {
-		f.spansAccountedFor = spans.MemUsage()
-		if err := f.acc.Grow(ctx, f.spansAccountedFor); err != nil {
+		newSpansAccountedFor := spans.MemUsage()
+		if err := f.acc.Grow(ctx, newSpansAccountedFor); err != nil {
 			return err
 		}
+		f.spansAccountedFor = newSpansAccountedFor
 	}
 
 	if spanIDs != nil && len(spans) != len(spanIDs) {