Destroy map iterators once we're done with them.

Each call to enif_map_iterator_create() must be paired with a call to
enif_map_iterator_destroy(). Otherwise, we'll leak memory.

Fixes #112

c_src/encoder.c

@@ -559,6 +559,7 @@ enc_map_to_ejson(ErlNifEnv* env, ERL_NIF_TERM map, ERL_NIF_TERM* out)
 
     do {
         if(!enif_map_iterator_get_pair(env, &iter, &key, &val)) {
+            enif_map_iterator_destroy(env, &iter);
             fprintf(stderr, "bad get pair\r\n");
             return 0;
         }
@@ -566,6 +567,8 @@ enc_map_to_ejson(ErlNifEnv* env, ERL_NIF_TERM map, ERL_NIF_TERM* out)
         list = enif_make_list_cell(env, tuple, list);
     } while(enif_map_iterator_next(env, &iter));
 
+    enif_map_iterator_destroy(env, &iter);
+
     *out = enif_make_tuple1(env, list);
     return 1;
 }