RFR: 8283325: US_ASCII decoder relies on String.decodeASCII being exhaustive

[Claes Redestad]

Adjust `String.decodeASCII` to deal with the possibility that `StringCoding.countPositives` can return a value less than the exact number of leading positive bytes. This is the likely cause of a number of intermittent but somewhat common errors on aarch64 tests in our CI.

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Commit messages:
 - Exhaustively consume ASCII bytes in String.decodeASCII

Changes: https://git.openjdk.java.net/jdk/pull/7855/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7855&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283325
  Stats: 57 lines in 2 files changed: 57 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7855.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7855/head:pull/7855

PR: https://git.openjdk.java.net/jdk/pull/7855