core/state: prevent build goroutine leak when SizeTracker is stopped

SizeTracker.build delivers its result on an unbuffered channel. If the
tracker is stopped while the initial measurement is running, init
returns via the abort arm without receiving, and the build goroutine
blocks forever on the send, regardless of whether the measurement
failed or completed.

Make both send sites select on the abort channel so the goroutine
always terminates.

core/state/state_sizer.go

@@ -538,7 +538,12 @@ func (t *SizeTracker) build(root common.Hash, blockNumber uint64, done chan buil
 
 	// Wait for all goroutines to complete
 	if err := group.Wait(); err != nil {
-		done <- buildResult{err: err}
+		select {
+		case done <- buildResult{err: err}:
+		case <-t.abort:
+			// The tracker was closed and the receiver in init has
+			// already exited, the result is no longer consumed.
+		}
 	} else {
 		stat := SizeStats{
 			StateRoot:            root,
@@ -554,11 +559,16 @@ func (t *SizeTracker) build(root common.Hash, blockNumber uint64, done chan buil
 			ContractCodes:        codes,
 			ContractCodeBytes:    codeBytes,
 		}
-		done <- buildResult{
+		select {
+		case done <- buildResult{
 			root:        root,
 			blockNumber: blockNumber,
 			stat:        stat,
 			elapsed:     time.Since(start),
+		}:
+		case <-t.abort:
+			// The tracker was closed and the receiver in init has
+			// already exited, the result is no longer consumed.
 		}
 	}
 }