Handle no async jobs in libssl

If the application has limited the size of the async pool using
ASYNC_init_thread() then we could run out of jobs while trying to start a
libssl io operation. However libssl was failing to handle this and treating
it like a fatal error. It should not be fatal...we just need to retry when
there are jobs available again.

Reviewed-by: Richard Levitte <levitte@openssl.org>
diff --git a/apps/s_server.c b/apps/s_server.c
index f0b28fd..9cbff09 100644
--- a/apps/s_server.c
+++ b/apps/s_server.c
@@ -2421,6 +2421,10 @@
                 case SSL_ERROR_WANT_X509_LOOKUP:
                     BIO_printf(bio_s_out, "Write BLOCK\n");
                     break;
+                case SSL_ERROR_WANT_ASYNC_JOB:
+                    /*
+                     * This shouldn't ever happen in s_server. Treat as an error
+                     */
                 case SSL_ERROR_SYSCALL:
                 case SSL_ERROR_SSL:
                     BIO_printf(bio_s_out, "ERROR\n");
@@ -2495,6 +2499,10 @@
                 case SSL_ERROR_WANT_READ:
                     BIO_printf(bio_s_out, "Read BLOCK\n");
                     break;
+                case SSL_ERROR_WANT_ASYNC_JOB:
+                    /*
+                     * This shouldn't ever happen in s_server. Treat as an error
+                     */
                 case SSL_ERROR_SYSCALL:
                 case SSL_ERROR_SSL:
                     BIO_printf(bio_s_out, "ERROR\n");