Use accessors for X509_STORE and X509_OBJECT

This will avoid build failures in the future when X509_STORE and
X509_OBJECT become opaque. Along the way, it does the following:

- Remove unnecessary NULL check for the object list (it's always
  non-NULL, and STACK_OF(T) functions generally treat NULL as the empty
  list).

- Use BoringSSL's ranged-for adapters for STACK_OF(T).

By using the X509_OBJECT accessor, rather than reaching into the union
directly, this also avoids a potential memory error in the future, if
you ever put non-X509 objects into your X509_STORE.
(X509_OBJECT_get0_X509 checks the type and returns NULL.)

TEST=rely on CI tests, this is refactor

Change-Id: Iafb8c06cfdfa86948119c229837b2e20b824f612
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/206460
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
Auto-Submit: David Benjamin <davidben@google.com>