[Coverity CID :186844] Parse warnings in /tests/net/ieee802154/fragment/src/main.c

[mandarcthorat1]

Static code scan issues seen in File: /tests/net/ieee802154/fragment/src/main.c
Category: Parse warnings
Function:
Component: Tests
CID: 186844
Please fix or provide comments to square it off in coverity in the link: https://scan9.coverity.com/reports.htm#v32951/p12996

[jukkar]

False positive.

[mandarcthorat1]

Fixed with #8645