tests: net: mqtt: Fix the error handling aftre read
tests: net: mqtt: Fix the error handling aftre read

If the rc = -EAGAIN from mqtt_read_publich_payload(), it shouldn't be
used in memcpy() since it is a negative value, and instead, it should
try to read again.

Fix: #13825
Coverity-CID: 191002

Signed-off-by: Tedd Ho-Jeong An <bccd9a0c49e2239629ed999837613a8438457215@intel.com>

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