Use the new argument ordering for mx_cprng_draw

runtime/bin/crypto_fuchsia.cc

Index: runtime/bin/crypto_fuchsia.cc
diff --git a/runtime/bin/crypto_fuchsia.cc b/runtime/bin/crypto_fuchsia.cc
index 04c5f2254760ee7b279625a1190d32a34b5ddcb4..0c23b720b9393316e4417478f9f868bc51a1e4c9 100644
--- a/runtime/bin/crypto_fuchsia.cc
+++ b/runtime/bin/crypto_fuchsia.cc
@@ -19,8 +19,9 @@ bool Crypto::GetRandomBytes(intptr_t count, uint8_t* buffer) {
     const intptr_t len =
         (MX_CPRNG_DRAW_MAX_LEN < remaining) ? MX_CPRNG_DRAW_MAX_LEN
                                             : remaining;
-    const mx_ssize_t res = mx_cprng_draw(buffer + read, len);
-    if (res == ERR_INVALID_ARGS) {
+    mx_size_t res;

[abarth, 2016/11/03 22:00:31]

I'd zero-initial because I'm paranoid.  Is ERR_INVALID_ARGS really the only
error that can cause this value not to be assigned?

[kulakowski1, 2016/11/03 22:08:56]

Done.

+    const mx_status_t status = mx_cprng_draw(buffer + read, len, &res);
+    if (status == ERR_INVALID_ARGS) {
       return false;
     }
     read += res;