Returns 0xF6 only if BAD_HOSTNAME

This commit modifies the behavior of `parse_extended_address` in such a
way that if it fails, it will always return a `BAD_HOSTNAME` value,
which is then used to return the 0xF6 extended error code.
This way, in any case that is not a valid v2 address, we return the 0xF6
error code, which is the expected behavior.
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