tests: socket: udp: Make sure client sockaddr fully initialized
tests: socket: udp: Make sure client sockaddr fully initialized

Previously, some fields like sin6_scope_id weren't explicitly
initialized. Such aren't really used, but to keep Coverity
happy, let's zero out the entire address structure.

Coverity-Id: 182763
Fixes: #6108

Signed-off-by: Paul Sokolovsky <35cb557f011771c94f127d6fef99f804b62203f5@linaro.org>

zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,explora26/zephyr,galak/zephyr,nashif/zephyr,kraj/zephyr,ldts/zephyr,explora26/zephyr,ldts/zephyr,zephyriot/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,mbolivar/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,mbolivar/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,kraj/zephyr,nashif/zephyr,kraj/zephyr,mbolivar/zephyr,punitvara/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,zephyriot/zephyr,ldts/zephyr,nashif/zephyr,punitvara/zephyr,punitvara/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyriot/zephyr