src: fix -Wempty-body compiler warnings

Turn counter macros into no-op instructions when counters are disabled.
Evaluating to nothing makes gcc complain when the macro is used in a
conditional.  Fixes the following warning:

    ../src/tls_wrap.cc:320:5: warning:
    suggest braces around empty body in an 'if' statement [-Wempty-body]
         NODE_COUNT_NET_BYTES_SENT(write_size_);
         ^

PR-URL: https://github.com/iojs/io.js/pull/974


Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>