fix possible nil before casting

[charlesxsh]

This PR aims to fix a possible nil before casting to error.

--- FAIL: TestClusterManagerForwardsBalancerBuildOptions (1.00s)
panic: interface conversion: interface is nil, not error [recovered]
	panic: interface conversion: interface is nil, not error

goroutine 93 [running]:
testing.tRunner.func1.2(0xceec40, 0xc00020f230)
	/usr/local/go/src/testing/testing.go:1143 +0x335
testing.tRunner.func1(0xc0003be280)
	/usr/local/go/src/testing/testing.go:1146 +0x4c2
panic(0xceec40, 0xc00020f230)
	/usr/local/go/src/runtime/panic.go:965 +0x1b9
google.golang.org/grpc/xds/internal/balancer/clustermanager.TestClusterManagerForwardsBalancerBuildOptions(0xc0003be280)
	/fuzz/target/xds/internal/balancer/clustermanager/clustermanager_test.go:635 +0x68f
testing.tRunner(0xc0003be280, 0xe8bbb0)
	/usr/local/go/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1238 +0x2b5

RELEASE NOTES: none

[dfawley]

@easwars it looks like you originally wrote this - what do you think?

[dfawley]

Isn't this actually a guarantee here?

I think this is written incorrectly, because it's not actually validating v at all in the normal case (where there is no timeout). Probably what was meant was:

v, err := ccsCh.Receive(ctx)
if err != nil {
	t.Fatalf("timed out waiting for UpdateClientConnState result: %v", err)
}
if v != nil {
	t.Fatal(v)
}

[charlesxsh]

Oh yes. We need to check err first.

[easwars]

@easwars it looks like you originally wrote this - what do you think?

Yup, good catch and fix I think.