[jdk8u-dev] RFR: 8301760: Fix possible leak in SpNegoContext dispose

[Yuri Nesterenko]

This is a small change fixing not very common but still existing in the wild issue in the stable code. Should not be risky.

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

Commit messages:
 - Backport 3f3356bcbd2b2fbc545263dc70a48ee931a4e56b

Changes: https://git.openjdk.org/jdk8u-dev/pull/265/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk8u-dev&pr=265&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/jdk8u-dev/pull/265.diff
  Fetch: git fetch https://git.openjdk.org/jdk8u-dev pull/265/head:pull/265

PR: https://git.openjdk.org/jdk8u-dev/pull/265