tar/asm: go vet fixes

on go1.19.7

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

tar/asm/disassemble_test.go

@@ -18,12 +18,13 @@ func TestLargeJunkPadding(t *testing.T) {
 	// Write a normal tar file into the pipe and then load it full of junk
 	// bytes as padding. We have to do this in a goroutine because we can't
 	// store 20GB of junk in-memory.
-	go func() {
+	var err error
+	go func(e error) {
 		// Empty archive.
 		tw := tar.NewWriter(pW)
 		if err := tw.Close(); err != nil {
 			pW.CloseWithError(err)
-			t.Fatal(err)
+			e = err
 			return
 		}
 
@@ -35,7 +36,7 @@ func TestLargeJunkPadding(t *testing.T) {
 		devZero, err := os.Open("/dev/zero")
 		if err != nil {
 			pW.CloseWithError(err)
-			t.Fatal(err)
+			e = err
 			return
 		}
 		defer devZero.Close()
@@ -45,14 +46,17 @@ func TestLargeJunkPadding(t *testing.T) {
 			}
 			if _, err := io.CopyN(pW, devZero, junkChunkSize); err != nil {
 				pW.CloseWithError(err)
-				t.Fatal(err)
+				e = err
 				return
 			}
 		}
 
 		fmt.Fprintln(os.Stderr, "[TestLargeJunkPadding] junk chunk finished")
 		pW.Close()
-	}()
+	}(err)
+	if err != nil {
+		t.Fatal(err)
+	}
 
 	// Disassemble our junk file.
 	nilPacker := storage.NewJSONPacker(ioutil.Discard)