RFR: 8266173: -Wmaybe-uninitialized happens in jni_util.c

[Yasumasa Suenaga]

We can see compiler warnings in jni_util.c as following on GCC 11. `buf` should be initialized.

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Commit messages:
 - 8266173: -Wmaybe-uninitialized happens in jni_util.c

Changes: https://git.openjdk.java.net/jdk/pull/3742/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3742&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266173
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3742.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3742/head:pull/3742

PR: https://git.openjdk.java.net/jdk/pull/3742