rwrap: Explicitly zero out sin_zero to silence gcc warnings.

rwrap: Explicitly zero out sin_zero to silence gcc warnings.

On some platforms, failure to set the sin_zero structure lead to
warnings such as:

resolv_wrapper.c:969:5: warning: missing initializer for field ‘sin_zero’
		        of ‘struct sockaddr_in’ [-Wmissing-field-initializers]

     };
     ^
In file included from /usr/include/arpa/inet.h:22:0,
                 from /.../source/src/resolv_wrapper.c:38:

Signed-off-by: Jakub Hrozek <jakub.hrozek@gmail.com>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Michael Adam <obnox@samba.org>