RFR: 8303809: Dispose context in SPNEGO NegotiatorImpl

[Alexey Bakhtin]

This patch fixes a possible native memory leak in case of a custom native GSS provider.
The actual leak was reported in production.

sun/security/jgss, sun/security/krb5, sun/net/www/protocol/http jtreg tests are passed

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Commit messages:
 - 8303809: Dispose context in SPNEGO NegotiatorImpl

Changes: https://git.openjdk.org/jdk/pull/12920/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12920&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303809
  Stats: 65 lines in 5 files changed: 65 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12920.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12920/head:pull/12920

PR: https://git.openjdk.org/jdk/pull/12920