Bluetooth: Mesh: Fix out of bounds write

Fix of the subnet bridging table function to only
compact the table if elements has been removed, and
fixing the compact function to compact the table if
several elemnts has been removed at the same time.

Fixes zephyrproject-rtos#78794

Signed-off-by: Ingar Kulbrandstad <ingar.kulbrandstad@nordicsemi.no>