Bluetooth: Mesh: Fix NULL pointer access with PB-ADV

When both PB-ADV and PB-GATT are enabled, the PB-ADV code (prov.c)
uses the bt_mesh_proxy_get_buf() API to get a net_buf_simple buffer.
Unfortunately this function also suffers from the same issue that was
fixed by commit 2b273444.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>