tls.Client: Reduce temporary buffer size for reads to prevent buffer overflow.

[hawkbee]

Resolves #15226, #15590.

[matu3ba]

Suggestions:

• Use as title tls.Client: Reduce temporary buffer size for reads to prevent buffer overflow.
• Explain how this solves the issues

[hawkbee]

Suggestions:

• Use as title tls.Client: Reduce temporary buffer size for reads to prevent buffer overflow.

ok.

• Explain how this solves the issues

I am not understand the code fully, When i test with another pull request #16114, my https test nearly panic ervery time.
I try to understand the TLS read flow, and see the buffer is 4 times of the TLS client buffer. If the underlying read return too many data, data copy may overflow, i guess.

After this change, no panic again, i had run test 200 times, all https download success.

This may be not the right way to solve the problem, but i had try my best to get here, and give others a hint to solve this problem.

[andrewrk]

Please explain why you believe this change to solve the problem, and what kinds of tests you performed to validate this change.

Edit: sorry I see that you did that above, in response to @matu3ba. I'm sorry but that is not a satisfactory answer. I will close this PR so that we can do more robust engineering via #14171 and use them to test each other.