uv: fix size calculation in select() fallback

Original commit message:

  darwin: fix size calculation in select() fallback

  Apple's `fd_set` stores its bits in an array of 32-bit integers, which
  means `FD_ISSET()` may read out of bounds if we allocate storage at
  byte granularity. There's also a chance that the `select()` call could
  corrupt the heap, although I didn't investigate that.

  This issue was discovered by LLVM's AddressSanitizer which caught
  `FD_ISSET()` trying to read out of bounds.

Ref: https://github.com/libuv/libuv/pull/241

Reviewed-By: Julien Gilli <julien.gilli@joyent.com>
PR-URL: https://github.com/joyent/node/pull/9179

deps/uv/src/unix/internal.h

@@ -55,6 +55,9 @@
 #define ACCESS_ONCE(type, var)                                                \
   (*(volatile type*) &(var))
 
+#define ROUND_UP(a, b)                                                        \
+  ((a) % (b) ? ((a) + (b)) - ((a) % (b)) : (a))
+
 #define UNREACHABLE()                                                         \
   do {                                                                        \
     assert(0 && "unreachable code");                                          \

deps/uv/src/unix/stream.c

@@ -301,7 +301,7 @@ int uv__stream_try_select(uv_stream_t* stream, int* fd) {
   if (fds[1] > max_fd)
     max_fd = fds[1];
 
-  sread_sz = (max_fd + NBBY) / NBBY;
+  sread_sz = ROUND_UP(max_fd + 1, sizeof(uint32_t) * NBBY) / NBBY;
   swrite_sz = sread_sz;
 
   s = malloc(sizeof(*s) + sread_sz + swrite_sz);