src: fix fs_stats_field_array memory leak

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#82

Author: addaleax

src/env-inl.h

@@ -332,6 +332,7 @@ 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

@@ -721,7 +721,7 @@ 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);