RFR: 8301760: Fix possible leak in SpNegoContext dispose

[Yuri Nesterenko]

This small change should fix a possible under certain circumstances native memory leak. In fact, the actual leak was reported in production.

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Commit messages:
 - 8301760: Fix possible leak in SpNegoContext dispose

Changes: https://git.openjdk.org/jdk/pull/12407/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12407&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301760
  Stats: 5 lines in 1 file changed: 4 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12407.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12407/head:pull/12407

PR: https://git.openjdk.org/jdk/pull/12407