Avoid memory leak within R_mongo_collection_find (#160)

This patch avoids the leakage by not allocating the memory in the first place but instead using `NULL` for `mongoc_collection_find_with_opts` to use the default values.

src/collection.c

@@ -173,8 +173,7 @@ SEXP R_mongo_collection_find(SEXP ptr_col, SEXP ptr_query, SEXP ptr_opts) {
   mongoc_collection_t *col = r2col(ptr_col);
   bson_t *query = r2bson(ptr_query);
   bson_t *opts = r2bson(ptr_opts);
-  mongoc_read_prefs_t * readpref = mongoc_read_prefs_new(MONGOC_READ_PRIMARY);
-  mongoc_cursor_t *c = mongoc_collection_find_with_opts(col, query, opts, readpref);
+  mongoc_cursor_t *c = mongoc_collection_find_with_opts(col, query, opts, NULL);
   return cursor2r(c, ptr_col);
 }