multi: add xfer_buf to multi handle

- can be borrowed by transfer during recv-write operation
- needs to be released before borrowing again
- adjustis size to `data->set.buffer_size`
- used in transfer.c readwrite_data()

Closes #12805

lib/easy.c

@@ -1021,7 +1021,6 @@ fail:
 #ifndef CURL_DISABLE_COOKIES
     free(outcurl->cookies);
 #endif
-    free(outcurl->state.buffer);
     Curl_dyn_free(&outcurl->state.headerb);
     Curl_altsvc_cleanup(&outcurl->asi);
     Curl_hsts_cleanup(&outcurl->hsts);