samples: net: sockets: echo-client: fix sock fd test, allow zero.
samples: net: sockets: echo-client: fix sock fd test, allow zero.

Per: http://pubs.opengroup.org/onlinepubs/009695399/functions/socket.html
"Upon successful completion, socket() shall return a non-negative
integer, the socket file descriptor."

The test in prepare_fds() however fails if socket fd is
zero (non-negative), which should be a valid value.

This was found while testing the SimpleLink socket offload driver,
which can return a zero-valued socket fd, per the POSIX spec.

Signed-off-by: Gil Pitney <477da50908f0a7963c2c490cce0ff096d2cac162@linaro.org>

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