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Fix for https://github.com/microsoft/mimalloc/issues/1138

diff --git a/src/arena.c b/src/arena.c
index b26f4442..d7e99b55 100644
--- a/src/arena.c
+++ b/src/arena.c
@@ -797,6 +797,9 @@ mi_page_t* _mi_arenas_page_alloc(mi_heap_t* heap, size_t block_size, size_t bloc
   else {
     page = mi_arenas_page_singleton_alloc(heap, block_size, block_alignment);
   }
+  if mi_unlikely(page == NULL) {
+    return NULL;
+  }
   // mi_assert_internal(page == NULL || _mi_page_segment(page)->subproc == tld->subproc);
   mi_assert_internal(_mi_is_aligned(page, MI_PAGE_ALIGN));
   mi_assert_internal(_mi_ptr_page(page)==page);