be explicit about memory tracking in os.c

src/prim/unix/prim.c

@@ -190,7 +190,11 @@ static void* unix_mmap_prim(void* addr, size_t size, size_t try_alignment, int p
     if (hint != NULL) {
       p = mmap(hint, size, protect_flags, flags, fd, 0);
       if (p==MAP_FAILED || !_mi_is_aligned(p,try_alignment)) { 
+        #if MI_TRACK_ENABLED  // asan sometimes does not instrument errno correctly?
+        int err = 0;
+        #else
         int err = errno;
+        #endif
         _mi_warning_message("unable to directly request hinted aligned OS memory (error: %d (0x%x), size: 0x%zx bytes, alignment: 0x%zx, hint address: %p)\n", err, err, size, try_alignment, hint);
       }
       if (p!=MAP_FAILED) return p;