Move to new mx_cprng_draw argument ordering

Change-Id: I4221f5301e04fdf2827bcedcdec4fede896f0d27
diff --git a/src/utils/os_fuchsia.c b/src/utils/os_fuchsia.c
index 97c97e6..fc73e14 100644
--- a/src/utils/os_fuchsia.c
+++ b/src/utils/os_fuchsia.c
@@ -153,16 +153,17 @@
   if (TEST_FAIL()) return -1;
 
   while (len > 0) {
-    mx_ssize_t output_bytes_this_pass = MX_CPRNG_DRAW_MAX_LEN;
+    mx_size_t output_bytes_this_pass = MX_CPRNG_DRAW_MAX_LEN;
     if (len < (size_t)output_bytes_this_pass) {
       output_bytes_this_pass = len;
     }
-    output_bytes_this_pass = mx_cprng_draw(buf, output_bytes_this_pass);
-    if (output_bytes_this_pass < 0) {
+    mx_size_t bytes_drawn;
+    mx_status_t status = mx_cprng_draw(buf, output_bytes_this_pass, &bytes_drawn);
+    if (status != NO_ERROR) {
       abort();
     }
-    len -= output_bytes_this_pass;
-    buf += output_bytes_this_pass;
+    len -= bytes_drawn;
+    buf += bytes_drawn;
   }
   return 0;
 }