Bluetooth: Mesh: Fix NULL pointer access with PB-ADV
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 2b273444c1230863aaaf613ae44bd74d86059204.

Signed-off-by: Johan Hedberg <628991c7b0a19c384f9b99c21ec330ce952bb838@intel.com>

Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,mbolivar/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,nashif/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,galak/zephyr,punitvara/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr,zephyriot/zephyr,mbolivar/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,kraj/zephyr,nashif/zephyr,zephyriot/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,kraj/zephyr,zephyriot/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,explora26/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,zephyriot/zephyr,punitvara/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,Vudentz/zephyr,explora26/zephyr,mbolivar/zephyr,nashif/zephyr,mbolivar/zephyr,explora26/zephyr,punitvara/zephyr