Add null check for return values of functions that are mostly (but not
always) checked for null.
E.g., since 98% of calls to protocol_stack_interface_info_get_by_id
check for null, it is likely that the function can return null values in
some cases, and omitting the check could crash the program.
```
connectivity
├── netsocket
├── lwipstack
├── nanostack
│ ├── mbed_lib.json // nanostack-interface's mbed_lib.json
│ ├── include
│ │ └── nanostack // headers from features/netsocket/nanostack-interface
│ ├── source // sources from features/netsocket/nanostack-interface
│ ├── coap-service // used by Thread only - deprecate?
│ ├── mbed-mesh-api
│ ├── nanostack-hal-mbed-cmsis-rtos // HAL porting layer for Nanostack on mbed with CMSIS-RTOS
│ ├── sal-stack-nanostack
│ └── sal-stack-nanostack-eventloop
├── cellular // previously in features/cellular/framework/
├── ...
```
This PR is a part of a wider \connectivity refactoring and implements the above new directory structure for \nanostack.
Mingjie Shen
Arto Kinnunen
Abbas Bracken Ziad
Ashok Rao