go-fuzz: don't drop data when Read also returns an error

The io.Reader contract specifies that Read may
return n > 0 and err != nil, and that in that scenario,
the caller should process those n bytes.

This is rare enough that it probably wasn't causing an actual problem.

Fix it nevertheless; one fewer latent bug.

go-fuzz/testee.go

@@ -198,9 +198,6 @@ retry:
 			case <-t.downC:
 			}
 			n, err := t.stdoutPipe.Read(data[filled:])
-			if err != nil {
-				break
-			}
 			if *flagV >= 3 {
 				log.Printf("testee: %v\n", string(data[filled:filled+n]))
 			}
@@ -209,6 +206,9 @@ retry:
 				copy(data, data[testeeBufferSize/2:filled])
 				filled -= testeeBufferSize / 2
 			}
+			if err != nil {
+				break
+			}
 		}
 		ticker.Stop()
 		trimmed := make([]byte, filled)