err: clear flags better when clearing errors.

An attempt to clear an error with malloced data didn't clear the flags.
Now it clears all flags except the malloced flag.

Fixes #12530

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15667)