tls: fix SecurePair external memory reporting

Ensure that AdjustAmountOfExternalAllocatedMemory() is called when
the SecurePair is destroyed.  Not doing so is not an actual memory
leak but it makes `process.memoryUsage().external` wildly inaccurate
and can cause performance problems due to excessive garbage collection.

PR-URL: https://github.com/nodejs/node/pull/11896


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>