unix: squelch harmless valgrind warning

Valgrind complains that the msg_control pointer points to uninitialized
memory.  While the memory is only used for writing data to, not for
reading data from, and the warning is therefore bogus, it's still
annoying enough that I decided to squelch it by zeroing the memory.

The performance implications should be minimal because this code path
is only used when sending over a handle to another process.

The warning:

    ==14859== Syscall param sendmsg(msg.msg_control) points to
    uninitialised byte(s)
      ==14859==    at 0x5AF1A80: __sendmsg_nocancel (in
          /usr/lib64/libpthread-2.21.so)
      ==14859==    by 0x46350E: uv__write (stream.c:810)
      ==14859==    by 0x464B24: uv_write2 (stream.c:1398)
      ==14859==    by 0x421ACE: run_test (test-ipc-send-recv.c:104)
      ==14859==    by 0x421DD1: run_test_ipc_send_recv_tcp
      (test-ipc-send-recv.c:156)
      ==14859==    by 0x406D2F: run_test_part (runner.c:404)
      ==14859==    by 0x4058CD: main (run-tests.c:58)
      ==14859==  Address 0xffefff934 is on thread 1's stack
      ==14859==  in frame #1, created by uv__write (stream.c:742)

PR-URL: https://github.com/libuv/libuv/pull/565
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/stream.c

@@ -779,9 +779,9 @@ start:
 
   if (req->send_handle) {
     struct msghdr msg;
-    char scratch[64];
     struct cmsghdr *cmsg;
     int fd_to_send = uv__handle_fd((uv_handle_t*) req->send_handle);
+    char scratch[64] = {0};
 
     assert(fd_to_send >= 0);