[release-branch.go1.11] crypto/x509: fix value ownership in isSSLPolicy on macOS

CFDictionaryGetValueIfPresent does not take ownership of the value, so
releasing the properties dictionary before passing the value to CFEqual
can crash. Not really clear why this works most of the time.

See https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html

Fixes #32281
Updates #28092
Updates #30763

Change-Id: I5ee7ca276b753a48abc3aedfb78b8af68b448dd4
Reviewed-on: https://go-review.googlesource.com/c/go/+/178537


Reviewed-by: Adam Langley <agl@golang.org>
(cherry picked from commit a3d4655c)
Reviewed-on: https://go-review.googlesource.com/c/go/+/179340


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>