[jdk21u-dev] 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.

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Commit messages:
 - Backport ea7e943874288e1cbea10a6bd82d6c7f2a1c9ae0

Changes: https://git.openjdk.org/jdk21u-dev/pull/2153/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=2153&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/jdk21u-dev/pull/2153.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/2153/head:pull/2153

PR: https://git.openjdk.org/jdk21u-dev/pull/2153