Bluetooth: Controller: Add ll_conn_get() return value check

Add ll_conn_get() return value check for valid connection
context.

Build command:
cmake -GNinja -DBOARD=nrf52833dk/nrf52833
 -DEXTRA_CONF_FILE=overlay-all-bt_ll_sw_split.conf
 -DDTC_OVERLAY_FILE=boards/nrf52833dk_nrf52833_df.overlay
 -DSNIPPET="bt-ll-sw-split" ../../samples/bluetooth/hci_uart
ninja

Before:
Memory region         Used Size  Region Size  %age Used
           FLASH:      283716 B       512 KB     54.11%
             RAM:      109752 B       128 KB     83.73%
        IDT_LIST:          0 GB        32 KB      0.00%

After:
Memory region         Used Size  Region Size  %age Used
           FLASH:      284992 B       512 KB     54.36%
             RAM:      109752 B       128 KB     83.73%
        IDT_LIST:          0 GB        32 KB      0.00%

After (use of `conn != NULL`):
Memory region         Used Size  Region Size  %age Used
           FLASH:      285044 B       512 KB     54.37%
             RAM:      109752 B       128 KB     83.73%
        IDT_LIST:          0 GB        32 KB      0.00%

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>