Fix hb_bytes_t.cmp() for realz this time

src/hb-dsalgs.hh

@@ -530,10 +530,12 @@ struct hb_bytes_t
 
   inline int cmp (const hb_bytes_t &a) const
   {
-    if (!len) return 0; /* glibc's memcmp() args are declared nonnull.  Meh. */
-
-    int r = memcmp (a.arrayZ, arrayZ, len);
-    if (r) return r;
+    unsigned int l = MIN(a.len, len);
+    if (l) /* glibc's memcmp() args are declared nonnull.  Meh. */
+    {
+      int r = memcmp (a.arrayZ, arrayZ, l);
+      if (r) return r;
+    }
 
     return a.len < len ? -1 : a.len > len ? +1 : 0;
   }