src: guard against double free in randomBytes()

PR-URL: #14122
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: bnoordhuis

src/node_crypto.cc

@@ -5498,6 +5498,7 @@ class RandomBytesRequest : public AsyncWrap {
     size_ = 0;
     if (free_mode_ == FREE_DATA) {
       free(data_);
+      data_ = nullptr;
     }
   }