[jdk25u] RFR: 8364257: JFR: User-defined events and settings with a one-letter name cannot be configured

[Rui Li]

A clean backport for https://bugs.openjdk.org/browse/JDK-8364257

This fixes a bug where a single letter name jfr event couldn't be configured. On tip for about one month. The test `test/jdk/jdk/jfr/api/flightrecorder/TestSettingsControl.java` passed locally. Other tests are running. Relatively low risk.

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

Commit messages:
 - Backport ea7e943874288e1cbea10a6bd82d6c7f2a1c9ae0

Changes: https://git.openjdk.org/jdk25u/pull/171/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk25u&pr=171&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364257
  Stats: 10 lines in 2 files changed: 4 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk25u/pull/171.diff
  Fetch: git fetch https://git.openjdk.org/jdk25u.git pull/171/head:pull/171

PR: https://git.openjdk.org/jdk25u/pull/171