http2: fix nil panic in typeFrameParser for unassigned frame types

The addition of FramePriorityUpdate (0x10) in RFC 9218 introduced a gap
in the frameParsers array indices (0x0a-0x0f). These indices were
initialized to nil, causing a panic when typeFrameParser accessed them
for unassigned frame types (e.g., ALTSVC 0x0a).

This change adds a nil check in typeFrameParser to safely fallback to
parseUnknownFrame for these unassigned types, preventing the crash.

Fixes golang/go#77652

Change-Id: I14d7ad85afc1eafabc46417a9fff10f9e0a22446
Reviewed-on: https://go-review.googlesource.com/c/net/+/746180
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>

http2/frame.go

@@ -145,7 +145,9 @@ var frameParsers = [...]frameParser{
 
 func typeFrameParser(t FrameType) frameParser {
 	if int(t) < len(frameParsers) {
-		return frameParsers[t]
+		if f := frameParsers[t]; f != nil {
+			return f
+		}
 	}
 	return parseUnknownFrame
 }

http2/frame_test.go

@@ -1585,3 +1585,20 @@ func TestReadFrameForHeaderUnexpectedEOF(t *testing.T) {
 		t.Fatalf("ReadFrameForHeader with short body = %v; want io.ErrUnexpectedEOF", err)
 	}
 }
+
+func TestTypeFrameParserHolePanic(t *testing.T) {
+	// Verify that unassigned frame types (0x0a-0x0f) don't panic. golang.org/issue/77652
+	fr, _ := testFramer()
+	if err := fr.WriteRawFrame(FrameType(0x0a), 0, 1, nil); err != nil {
+		t.Fatal(err)
+	}
+
+	f, err := fr.ReadFrame()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, ok := f.(*UnknownFrame); !ok {
+		t.Errorf("got %T; want *UnknownFrame", f)
+	}
+}