net: lwm2m: fix json formatter putchar check
net: lwm2m: fix json formatter putchar check

Found via Coverity CID 191001: Control flow issues  (NO_EFFECT)
This less-than-zero comparison of an unsigned value is never true:
"put_char(out, '}') < 0U".

Let's fix this check to be less than 1 instead as it should have
been originally.

Signed-off-by: Michael Scott <a17fed27eaa842282862ff7c1b9c8395a26ac320@foundries.io>

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