Fixed incorrect rollover after copy

If the copy operation ended at the last spot in a write buffer, it would be equal to `size` instead of rolled back to zero. 

Now while copying bytes, watches for rollover after the copy operation.

lwrb/src/lwrb/lwrb_ex.c

@@ -135,14 +135,13 @@ size_t lwrb_copy(lwrb_t* dest, lwrb_t* src)
     for (i = 0; i < len_to_copy; i++)
     {
 
+        dest->buff[dest_w++] = src->buff[src_r++];
+
         /* Handle roll-over / wrap for both source and destination indexes */
         if (dest_w >= dest->size)
             dest_w = 0;
-
         if (src_r >= src->size)
             src_r = 0;
-
-        dest->buff[dest_w++] = src->buff[src_r++];
    }
 
     atomic_store_explicit(&dest->w, dest_w, memory_order_release);