Reapply possible bsearch overflow fix

Originally introduced in 21ede867 (#1314) but as it wasn't applied to hb_bsearch
accidentally removed while merging hb_bsearch_r to it.

src/hb-algs.hh

@@ -613,7 +613,7 @@ hb_bsearch (const void *key, const void *base,
   int min = 0, max = (int) nmemb - 1;
   while (min <= max)
   {
-    int mid = (min + max) / 2;
+    int mid = ((unsigned int) min + (unsigned int) max) / 2;
     const void *p = (const void *) (((const char *) base) + (mid * size));
     int c = compar (key, p, ds...);
     if (c < 0)