tests: net: socket: Fix -Wsometimes-uninitialized warning

Building with clang warns:

tests/net/socket/tcp/src/main.c:377:13: error: variable 'c_sock' is used
uninitialized whenever 'if' condition is false
[-Werror,-Wsometimes-uninitialized]
        } else if (family == AF_INET6) {
                   ^~~~~~~~~~~~~~~~~~
tests/net/socket/tcp/src/main.c:396:15: note: uninitialized use occurs here
        test_connect(c_sock, s_saddr, addrlen);
                     ^~~~~~
tests/net/socket/tcp/src/main.c:377:9: note: remove the 'if' if its
condition is always true
        } else if (family == AF_INET6) {
               ^~~~~~~~~~~~~~~~~~~~~~~~
tests/net/socket/tcp/src/main.c:360:12: note: initialize the variable
'c_sock' to silence this warning
        int c_sock;
                  ^
                   = 0
tests/net/socket/tcp/src/main.c:377:13: error: variable 's_sock' is used
uninitialized whenever 'if' condition is false
[-Werror,-Wsometimes-uninitialized]
        } else if (family == AF_INET6) {
                   ^~~~~~~~~~~~~~~~~~
tests/net/socket/tcp/src/main.c:387:12: note: uninitialized use occurs here
        test_bind(s_sock, s_saddr, addrlen);
                  ^~~~~~
tests/net/socket/tcp/src/main.c:377:9: note: remove the 'if' if its
condition is always true
        } else if (family == AF_INET6) {
               ^~~~~~~~~~~~~~~~~~~~~~~~
tests/net/socket/tcp/src/main.c:361:12: note: initialize the variable
's_sock' to silence this warning
        int s_sock;
                  ^
                   = 0

Not really needed since we have zassert_unreachable(), but doesn't hurt
to initialize the variables.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>