tlsio_openssl_create uses malloc + per-member init; calloc would be more robust to future members

### Summary

`tlsio_openssl_create` allocates `TLS_IO_INSTANCE` with `malloc` and then initialises each member individually. This is correct today, but it is fragile: a member added later without a matching assignment would be left uninitialised, and nothing catches that at compile time.

`adapters/tlsio_openssl.c`, at `03fa57c6`:

```c
result = malloc(sizeof(TLS_IO_INSTANCE));
```
(line 1288, with the per-member assignments at lines 1293 and 1328-1363)

### Current state

Not a live defect. I checked every member of `TLS_IO_INSTANCE`: all 26 are assigned before the instance is returned, and both early-failure paths free the allocation and return `NULL` before any member is read. Nothing is read uninitialised on any path today.

### Suggested change

Use `calloc(1, sizeof(TLS_IO_INSTANCE))`, keeping the explicit assignments. This makes the zero-initialised state the default, so a member introduced later is `NULL`/`0` rather than indeterminate. The cost is one zeroing of a small struct at create time, off any hot path.

### Notes

- Purely defensive; no behaviour change and no functional bug is being reported.
- The many `= NULL` / `= false` assignments could optionally be dropped afterwards, though keeping them is also fine and makes the intent explicit.
- Same pattern is worth a look in the other TLS adapters if this is taken.