fix(picodata): don't close stderr & stdout in log_destroy

Previously, log_destroy would close log->fd even if it's one of the
standard streams. This behavior almost never makes sense, unless
one's trying to write a unix daemon which is obviously not the case.

Closing stderr has the following side effects:
- it breaks a reinit of the default logger;
- it inhibits ASan's final leak report;
- it causes a EPOLLHUP during a restart via `--entrypoint-fd` (Picodata).

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NO_TEST=internal
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