tests: lib: cbprintf_package: Fix issue reported by coverity
tests: lib: cbprintf_package: Fix issue reported by coverity

Fixing issue 219727. Potentially negative return value was passed
to a function that was expecting positive value.

Signed-off-by: Krzysztof Chruscinski <9b6dc988558c51b0405b49b55c8e66862969844b@nordicsemi.no>

finikorg/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr