Ensure |rwstate| is set correctly on BIO_flush

A BIO_flush call in the DTLS code was not correctly setting the |rwstate|
variable to SSL_WRITING. This means that SSL_get_error() will not return
SSL_ERROR_WANT_WRITE in the event of an IO retry.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

ssl/statem/statem_dtls.c

@@ -250,6 +250,8 @@ int dtls1_do_write(SSL *s, int type)
         blocksize = 0;
 
     frag_off = 0;
+    s->rwstate = SSL_NOTHING;
+
     /* s->init_num shouldn't ever be < 0...but just in case */
     while (s->init_num > 0) {
         if (type == SSL3_RT_HANDSHAKE && s->init_off != 0) {
@@ -298,8 +300,10 @@ int dtls1_do_write(SSL *s, int type)
              * grr.. we could get an error if MTU picked was wrong
              */
             ret = BIO_flush(SSL_get_wbio(s));
-            if (ret <= 0)
+            if (ret <= 0) {
+                s->rwstate = SSL_WRITING;
                 return ret;
+            }
             used_len = DTLS1_RT_HEADER_LENGTH + mac_size + blocksize;
             if (s->d1->mtu > used_len + DTLS1_HM_HEADER_LENGTH) {
                 curr_mtu = s->d1->mtu - used_len;