cares: fix uninitialized variable warning

Suppress the following warning:

  ../../src/cares_wrap.cc: In function ‘v8::Handle<v8::Value>
  node::cares_wrap::SetServers(const v8::Arguments&)’:
  ../../src/cares_wrap.cc:1017:5: warning: ‘uv_ret.uv_err_s::code’
  may be used uninitialized in this function [-Wuninitialized]