Avoid an undefined behavior of memcpy() in _tr_stored_block().

Allegedly the behavior of memcpy() is undefined if the source
pointer is NULL, even if the number of bytes to copy is zero.

trees.c

@@ -870,6 +870,7 @@ void ZLIB_INTERNAL _tr_stored_block(s, buf, stored_len, last)
     bi_windup(s);        /* align on byte boundary */
     put_short(s, (ush)stored_len);
     put_short(s, (ush)~stored_len);
+    if (stored_len)
         zmemcpy(s->pending_buf + s->pending, (Bytef *)buf, stored_len);
     s->pending += stored_len;
 #ifdef ZLIB_DEBUG