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CreateAuthenticator() relies on VirtualAuthenticator IDs being unique,
which they are. But if they weren't, it would first dereference and then
return a dangling VirtualAuthenticator pointer. Since a similar code
piece has recently shown that same bug, make this method more defensive
so it doesn't regress under future changes.

Also fix a possible nullptr dereference.

Change-Id: Ia769929f94360eabcdadb5e0f706f83d031231f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209905
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Auto-Submit: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Nina Satragno <nsatragno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770768}