RFR: 8261404: Class.getReflectionFactory() is not thread-safe

[liach]

Simply changes this to only read the static field once to prevent `null` on second read.

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Commit messages:
 - 8261404: Class.getReflectionFactory() is not thread-safe

Changes: https://git.openjdk.java.net/jdk/pull/6870/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6870&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261404
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6870.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6870/head:pull/6870

PR: https://git.openjdk.java.net/jdk/pull/6870