[subset-cff] Micro-optimize copy_str more

src/hb-subset-cff-common.hh

@@ -120,8 +120,11 @@ struct str_encoder_t
 
     /* Faster than hb_memcpy for small strings. */
     auto arr = buff.arrayZ + offset;
-    for (unsigned i = 0; i < length; i++)
+    /* Length is at least one; and mostly just one. */
+    arr[0] = str[0];
+    for (unsigned i = 1; i < length; i++)
       arr[i] = str[i];
+
     //hb_memcpy (buff.arrayZ + offset, str, length);
   }