Fix an s_server infinite loop

Commit c4666bfa changed s_server so that it asked libssl rather than the
underlying socket whether an error is retryable or not on the basis that
libssl has more information. That is true unfortunately the method used
was wrong - it only checks libssl's own internal state rather than both
libssl and the BIO. Should use SSL_get_error() instead.

This issue can cause an infinite loop because some errors could appear as
retryable when in fact they are not.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NTim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3317)

apps/s_server.c

@@ -2609,6 +2609,16 @@ static void close_accept_socket(void)
     }
 }
 
+static int is_retryable(SSL *con, int i)
+{
+    int err = SSL_get_error(con, i);
+
+    /* If it's not a fatal error, it must be retryable */
+    return (err != SSL_ERROR_SSL)
+           && (err != SSL_ERROR_SYSCALL)
+           && (err != SSL_ERROR_ZERO_RETURN);
+}
+
 static int init_ssl_connection(SSL *con)
 {
     int i;
@@ -2651,7 +2661,7 @@ static int init_ssl_connection(SSL *con)
         i = SSL_accept(con);
 
         if (i <= 0)
-            retry = !SSL_want_nothing(con);
+            retry = is_retryable(con, i);
 #ifdef CERT_CB_TEST_RETRY
         {
             while (i <= 0
@@ -2661,7 +2671,7 @@ static int init_ssl_connection(SSL *con)
                            "LOOKUP from certificate callback during accept\n");
                 i = SSL_accept(con);
                 if (i <= 0)
-                    retry = !SSL_want_nothing(con);
+                    retry = is_retryable(con, i);
             }
         }
 #endif
@@ -2682,7 +2692,7 @@ static int init_ssl_connection(SSL *con)
                 BIO_printf(bio_s_out, "LOOKUP not successful\n");
             i = SSL_accept(con);
             if (i <= 0)
-                retry = !SSL_want_nothing(con);
+                retry = is_retryable(con, i);
         }
 #endif
     } while (i < 0 && SSL_waiting_for_async(con));