client: fix Agent use after free

[trim21]

Please provide enough information so that others can review your pull request:

test may fail with race problem.

==================
--- FAIL: Test_Client_Unsupported_Protocol (0.00s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Agent_Custom_Response (0.01s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Ctx_ClientHelloInfo (0.00s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Agent_RetryIf (0.00s)
    testing.go:1319: race detected during execution of test
    --- FAIL: Test_Client_Agent_Dest/enough_dest (0.00s)
        testing.go:1319: race detected during execution of test
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Agent_MaxRedirectsCount (0.01s)
    --- FAIL: Test_Client_Agent_MaxRedirectsCount/error (0.00s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Put (0.01s)
--- FAIL: Test_Client_Patch (0.01s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Agent_Struct (0.00s)
--- FAIL: Test_Client_Delete (0.00s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Post (0.00s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Head (0.00s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_UserAgent (0.01s)
    --- FAIL: Test_Client_UserAgent/default (0.01s)
        testing.go:1319: race detected during execution of test
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Agent_InsecureSkipVerify (0.01s)
    testing.go:1319: race detected during execution of test
--- FAIL: Test_Client_Agent_Timeout (0.31s)
FAIL

Explain the details for making this change. What existing problem does the pull request solve?

Use After Free:

Agent.Struct may read Agent.jsonDecoder after agent.Bytes calling agent.release

[wangjq4214]

The client is being refactored and I will avoid these issues in my commit. Maybe it can be closed?

[efectn]

Perhaps master would be better for this PR

[wangjq4214]

Perhaps master would be better for this PR

Yes, I think so, changing target branch is better.

[efectn]

Can you rebase 😅😅

[trim21]

done

[trim21]

can we merge this into v3 too? current flaky test is a little bit annoying

[efectn]

can we merge this into v3 too? current flaky test is a little bit annoying

i'll merge it asap