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: #974
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>

src/node_counters.h

@@ -7,18 +7,18 @@
 #include "node_win32_perfctr_provider.h"
 #else
 #define NODE_COUNTER_ENABLED() (false)
-#define NODE_COUNT_HTTP_SERVER_REQUEST()
-#define NODE_COUNT_HTTP_SERVER_RESPONSE()
-#define NODE_COUNT_HTTP_CLIENT_REQUEST()
-#define NODE_COUNT_HTTP_CLIENT_RESPONSE()
-#define NODE_COUNT_SERVER_CONN_OPEN()
-#define NODE_COUNT_SERVER_CONN_CLOSE()
-#define NODE_COUNT_NET_BYTES_SENT(bytes)
-#define NODE_COUNT_NET_BYTES_RECV(bytes)
-#define NODE_COUNT_GET_GC_RAWTIME()
-#define NODE_COUNT_GC_PERCENTTIME(percent)
-#define NODE_COUNT_PIPE_BYTES_SENT(bytes)
-#define NODE_COUNT_PIPE_BYTES_RECV(bytes)
+#define NODE_COUNT_GC_PERCENTTIME(percent) do { } while (false)
+#define NODE_COUNT_GET_GC_RAWTIME() do { } while (false)
+#define NODE_COUNT_HTTP_CLIENT_REQUEST() do { } while (false)
+#define NODE_COUNT_HTTP_CLIENT_RESPONSE() do { } while (false)
+#define NODE_COUNT_HTTP_SERVER_REQUEST() do { } while (false)
+#define NODE_COUNT_HTTP_SERVER_RESPONSE() do { } while (false)
+#define NODE_COUNT_NET_BYTES_RECV(bytes) do { } while (false)
+#define NODE_COUNT_NET_BYTES_SENT(bytes) do { } while (false)
+#define NODE_COUNT_PIPE_BYTES_RECV(bytes) do { } while (false)
+#define NODE_COUNT_PIPE_BYTES_SENT(bytes) do { } while (false)
+#define NODE_COUNT_SERVER_CONN_CLOSE() do { } while (false)
+#define NODE_COUNT_SERVER_CONN_OPEN() do { } while (false)
 #endif
 
 #include "v8.h"