Fix mi_page_is_used_at_frac for pages that have few allocatable blocks

include/mimalloc/internal.h

diff --git a/include/mimalloc/internal.h b/include/mimalloc/internal.h
index 5b766d7..7dd154c 100644
--- a/include/mimalloc/internal.h
+++ b/include/mimalloc/internal.h
@@ -697,6 +697,7 @@
 // is more than (n-1)/n'th of a page in use?
 static inline bool mi_page_is_used_at_frac(const mi_page_t* page, uint16_t n) {
   if (page==NULL) return true;
+  if (page->reserved < n) return false;
   uint16_t frac = page->reserved / n;
   return (page->reserved - page->used <= frac);
 }