Found by -fsanitize=address

```
=================================================================
==35428==ERROR: AddressSanitizer: global-buffer-overflow on
address 0x0000021039a0 at pc 0x000000566dd6 bp 0x7ffc7974ccd0 sp
0x7ffc7974ccc0
WRITE of size 4 at 0x0000021039a0 thread T0
    #0 0x566dd5 in main
    targets/RT/USER/lte-uesoftmodem.c:629
    #1 0x7f36223cb83f in __libc_start_main
    (/lib/x86_64-linux-gnu/libc.so.6+0x2083f)
    #2 0x582218 in _start
    (cmake_targets/ran_build/build/lte-uesoftmodem+0x582218)
```