actually increase stralloc.len

buffer/buffer_tosa.c

@@ -4,6 +4,7 @@
 static ssize_t strallocwrite(int fd,char* buf,size_t len,void* myself) {
   buffer* b=myself;
   stralloc* sa=b->cookie;
+  sa->len+=len;
   if (stralloc_readyplus(sa,1024)==0) return 0;
   b->x=sa->s+sa->len;
   b->p=0;