http2: fix nil dereference after Read completes with an error

Case happens if Read is called after it has already returned an error
previously. Verified that the new TestPipeCloseWithError test fails
before this change but passes after.

Updates golang/go#20501

Change-Id: I636fbb194f2d0019b0722556cc25a88da2d18e13
Reviewed-on: https://go-review.googlesource.com/44330
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/pipe.go

@@ -50,7 +50,7 @@ func (p *pipe) Read(d []byte) (n int, err error) {
 		if p.breakErr != nil {
 			return 0, p.breakErr
 		}
-		if p.b.Len() > 0 {
+		if p.b != nil && p.b.Len() > 0 {
 			return p.b.Read(d)
 		}
 		if p.err != nil {

http2/pipe_test.go

@@ -92,9 +92,12 @@ func TestPipeCloseWithError(t *testing.T) {
 	if err != a {
 		t.Logf("read error = %v, %v", err, a)
 	}
-	// Write should fail.
+	// Read and Write should fail.
 	if n, err := p.Write([]byte("abc")); err != errClosedPipeWrite || n != 0 {
-		t.Errorf("Write(abc) after close\ngot =%v, %v\nwant 0, %v", n, err, errClosedPipeWrite)
+		t.Errorf("Write(abc) after close\ngot %v, %v\nwant 0, %v", n, err, errClosedPipeWrite)
+	}
+	if n, err := p.Read(make([]byte, 1)); err == nil || n != 0 {
+		t.Errorf("Read() after close\ngot %v, nil\nwant 0, %v", n, errClosedPipeWrite)
 	}
 }
 
@@ -115,9 +118,13 @@ func TestPipeBreakWithError(t *testing.T) {
 	}
 	// Write should succeed silently.
 	if n, err := p.Write([]byte("abc")); err != nil || n != 3 {
-		t.Errorf("Write(abc) after break\ngot =%v, %v\nwant 0, nil", n, err)
+		t.Errorf("Write(abc) after break\ngot %v, %v\nwant 0, nil", n, err)
 	}
 	if p.b != nil {
 		t.Errorf("buffer should be nil after Write")
 	}
+	// Read should fail.
+	if n, err := p.Read(make([]byte, 1)); err == nil || n != 0 {
+		t.Errorf("Read() after close\ngot %v, nil\nwant 0, not nil", n)
+	}
 }