RFR: 8325002: Exceptions::fthrow needs to ensure it truncates to a valid utf8 string [v4]

[David Holmes]

> Exceptions::fthrow uses a 1024 byte buffer to format the incoming exception message string, but this may not be large enough, leading to truncation. However, we should ensure we truncate to a valid UTF8 sequence.
> 
> The process is explained in the code. Thanks to @RogerRiggs and @djelinski for their suggestions on how to tackle this.
> 
> Testing:
>  - new gtest exercises the truncation code with the different possibilities for bad truncation
>  - tiers 1-3 sanity testing
> 
> Thanks.

David Holmes has updated the pull request incrementally with one additional commit since the last revision:

  Fix logic to allow for the buffer being partially filled.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20345/files
  - new: https://git.openjdk.org/jdk/pull/20345/files/794da826..77079058

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20345&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20345&range=02-03

  Stats: 11 lines in 1 file changed: 8 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20345.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20345/head:pull/20345

PR: https://git.openjdk.org/jdk/pull/20345