RFR: 8303809: Dispose context in SPNEGO NegotiatorImpl [v2]

[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

Alexey Bakhtin has updated the pull request incrementally with one additional commit since the last revision:

  Fix formatting

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12920/files
  - new: https://git.openjdk.org/jdk/pull/12920/files/8efa0c76..1b34252b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12920&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12920&range=00-01

  Stats: 7 lines in 3 files changed: 5 ins; 0 del; 2 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