disallow further arena allocation if OS allocation is not allowed (issues #1218)

src/arena.c

diff --git a/src/arena.c b/src/arena.c
index 36f2c8d..1de14db 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -498,6 +498,9 @@
   // don't create arena's while preloading (todo: or should we?)
   if (_mi_preloading()) return NULL;
 
+  // don't create arena's if OS allocation is disallowed
+  if (mi_option_is_enabled(mi_option_disallow_os_alloc)) return NULL;
+
   // otherwise, try to reserve a new arena -- but one thread at a time.. (todo: allow 2 or 4 to reduce contention?)
   mi_subproc_t* const subproc = heap->subproc;
   const size_t arena_count = mi_arenas_get_count(subproc);