[Fix] Fix memory leak in upstream address parsing

[Fix] Fix memory leak in upstream address parsing

When parsing upstream addresses in rspamd_upstreams_add_upstream(),
the GPtrArray 'addrs' was not properly freed in several cases:

1. For service= variant: destructor was not added to mempool
2. After copying addresses to upstream: array was not freed when
   ups->ctx is NULL
3. On parse failure: addrs was not freed

This commit adds proper cleanup:
- Add mempool destructor for service= variant when ctx exists
- Free addrs after copying addresses if no ctx (not managed by mempool)
- Free addrs on parse failure if not managed by mempool
- Set addrs = NULL after freeing to avoid dangling pointer

The fix is careful about mempool-managed vs manual cleanup to avoid
double-free issues, especially important since upstreams can be called
from destructor functions.