Fix go vet errors

Docker-DCO-1.1-Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com> (github: LK4D4)

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)

engine/streams_test.go

@@ -141,7 +141,7 @@ func TestOutputAdd(t *testing.T) {
 		t.Fatalf("Expected %d, got %d", len(input), n)
 	}
 	if output := b.String(); output != input {
-		t.Fatal("Received wrong data from Add.\nExpected: '%s'\nGot:     '%s'", input, output)
+		t.Fatalf("Received wrong data from Add.\nExpected: '%s'\nGot:     '%s'", input, output)
 	}
 }
 

engine/table.go

@@ -137,5 +137,4 @@ func (t *Table) ReadFrom(src io.Reader) (n int64, err error) {
 		}
 		t.Add(env)
 	}
-	return 0, nil
 }