routerrpc: Check for nil IncomingCircuitKey

The grpc stream.Recv() will decode an ForwardHtlcInterceptResponse
without an IncomingCircuit, which will leave that pointer nil,
causing a nil pointer dereferece when components of in.IncomingCircuitKey
are used in the resolveFromClient() function.

This commit checks for the nil pointer and returns an error before
the rest of the parsing.

docs/release-notes/release-notes-0.16.0.md

@@ -125,6 +125,9 @@ current gossip sync query status.
   update in order to use the new versioned RPC and upgrade any on-chain outputs
   to the new version.
 
+* [A bug has been fixed which could cause `lnd` to crash when parsing a
+  malformed HTLC intercept message](https://github.com/lightningnetwork/lnd/pull/7392).
+
 ## Wallet
 
 * [Allows Taproot public keys and tap scripts to be imported as watch-only

lnrpc/routerrpc/forward_interceptor.go

@@ -100,6 +100,11 @@ func (r *forwardInterceptor) onIntercept(
 func (r *forwardInterceptor) resolveFromClient(
 	in *ForwardHtlcInterceptResponse) error {
 
+	if in.IncomingCircuitKey == nil {
+		return status.Errorf(codes.InvalidArgument,
+			"CircuitKey missing from ForwardHtlcInterceptResponse")
+	}
+
 	log.Tracef("Resolving intercepted packet %v", in)
 
 	circuitKey := models.CircuitKey{