RFR: JDK-8260030: Improve stringStream buffer handling [v2]

[Thomas Stuefe]

On Thu, 21 Jan 2021 14:44:06 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Kim Feedback
>
> Changes requested by kbarrett (Reviewer).

I rewrote stringStream::write somewhat:
- I check and ignore len==0
- I check len for unreasonably high values (>1G). I assert, in release I return: stringStream is used for tracing a lot and we never touch all tracing code in debug. I did not add a overflow check, though I could, since overflow would require very high values for _written (> SIZE_MAX - 1G) which is not possible.
- I changed the guarantee to an assert, and added a similar assert to the start of the function

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PR: https://git.openjdk.java.net/jdk/pull/2160