Segmetation Fault Occurs on Downlink

Hi.

I'm trying to run mbed-os-example-lorawan with B-L072Z-LRWAN1 board.
only changes are some configurations on `mbed_app.json`

```json
"target_overrides": {
"*": {
"platform.stdio-convert-newlines": true,
"platform.stdio-baud-rate": 115200,
"platform.default-serial-baud-rate": 115200,
"lora.over-the-air-activation": true,
"lora.duty-cycle-on": false,
"target.components_add": ["SX1272", "SX1276", "SX126X"],
"lora.phy": "KR920",
"lora.device-eui": "(My 8-bytes device-eui key)",
"lora.application-eui": "(My 8-bytes application-eui)",
"lora.application-key": "(My 16-bytes application-key)"
},
```

When downlink from TTN occurs, the whole program stops.
Running debug, I got this debug output.


```
Attached to debugger on port 50000
0003295:INFO:loader:Erased 0 bytes (0 sectors), programmed 0 bytes (0 pages), skipped 100480 bytes (785 pages) at 44.78 kB/s
Image loaded: \Users\vkdls\Mbed Programs\mbed-os-example-lorawan2\BUILD\DISCO_L072CZ_LRWAN1\ARMC6\mbed-os-example-lorawan2.elf
Note: automatically using hardware breakpoints for read-only addresses.
[New Thread 536878128]
[New Thread 536878196]
[New Thread 536878264]
[New Thread 536881424]
[Switching to Thread 536878128]
Thread 2 "main" hit Breakpoint 1, main () at ./main.cpp:96
96 setup_trace();
[New Thread 2]
Thread
6 "Handler mode" received signal SIGSEGV, Segmentation fault.
[Switching to Thread 2]
0x080001c4 in HardFault_Handler ()
```

mbed-os: `890f056`
example: `0048d61`

Is there any solution?

Thanks.

Start with main.cpp at line 96 and reproduce the downlink failure using the supplied mbed_app.json overrides, board, and mbed OS/example revisions. Use the debugger output around HardFault_Handler to identify the failing path; done means the example continues running when a TTN downlink arrives without a HardFault.

Written by the indexing model from the issue text.