Fix ssl3_do_write() to correctly handle retries

A BIO is documented to return -1 on write retry - but sometimes they return
0. ssl3_do_write() was incorrectly handling a 0 response.

Fixes openssl#21422

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from openssl#21435)

(cherry picked from commit 034ea1d)

ssl/statem/statem_lib.c

@@ -45,7 +45,7 @@ int ssl3_do_write(SSL *s, int type)
 
     ret = ssl3_write_bytes(s, type, &s->init_buf->data[s->init_off],
                            s->init_num, &written);
-    if (ret < 0)
+    if (ret <= 0)
         return -1;
     if (type == SSL3_RT_HANDSHAKE)
         /*