Bluetooth: Mesh: Fix build warning with gcc 13

gcc 13 thinks  max_adv_duration may be used unitialized
and warns accordingly (see below)
It seems the reason is the goto, which confuses it.
In any case, pacifying this warning is trivial,
so let's just do so.

The warning:
```
In function ‘gatt_proxy_advertise’,
inlined from ‘bt_mesh_proxy_adv_start’ at
subsys/bluetooth/mesh/proxy_srv.c:1214:9:
subsys/bluetooth/mesh/proxy_srv.c:842:44: error: ‘max_adv_duration’ may
be used uninitialized [-Werror=maybe-uninitialized]
subsys/bluetooth/mesh/proxy_srv.c: In function ‘bt_mesh_proxy_adv_start’
zephyr/subsys/bluetooth/mesh/proxy_srv.c:786:17: note:
‘max_adv_duration’ was declared here
  786 |         int32_t max_adv_duration;
      |                 ^~~~~~~~~~~~~~~~
```

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>