bridge: Fix warning about uninitialized value
bridge: Fix warning about uninitialized value

Initialize the socket auto pointer right away. This fixes this warning:

```
src/bridge/cockpitdbusinternal.c: In function 'create_io_stream_for_unix_socket':
/usr/include/glib-2.0/glib/gmacros.h:1049:17: warning: 'socket' may be used uninitialized in this function [-Wmaybe-uninitialized]
 1049 |     { if (_ptr) (cleanup) ((ParentName *) _ptr); }                                                              \
      |                 ^
src/bridge/cockpitdbusinternal.c:66:22: note: 'socket' was declared here
   66 |   g_autoptr(GSocket) socket;
      |                      ^~~~~~
```

Closes #14649
martinpitt/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,garrett/cockpit,deryni/cockpit,garrett/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,garrett/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,mvollmer/cockpit,mvollmer/cockpit,deryni/cockpit,deryni/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,garrett/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit