src: fix fs_stats_field_array memory leak

Original PR: ayojs/ayo#82

> Previously, the `Environment` destructor did not release its
> `fs_stats_field_array` memory. The memory is allocated when the
> `fs` module is initialized and calls `GetStatValues()`.

> PR-URL: ayojs/ayo#82
> Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>

Author: addaleax

src/env-inl.h

@@ -334,6 +334,7 @@ inline Environment::~Environment() {
   delete[] heap_space_statistics_buffer_;
   delete[] http_parser_buffer_;
   delete http2_state_;
+  delete[] fs_stats_field_array_;
   free(performance_state_);
 }
 

src/env.h

@@ -723,7 +723,7 @@ class Environment {
   char* http_parser_buffer_;
   http2::http2_state* http2_state_ = nullptr;
 
-  double* fs_stats_field_array_;
+  double* fs_stats_field_array_ = nullptr;
 
   struct AtExitCallback {
     void (*cb_)(void* arg);