Fix mis-sized buffer

Reported by rgacogne on twitter.
Also spotted by gcc-4.8 with -O2
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 115bf0a..dd31a64 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -254,7 +254,7 @@
     unsigned char *enc_len_p = iv + 16;
     unsigned char *ticket = enc_len_p + 2;
     unsigned char *mac;
-    unsigned char computed_mac[16];
+    unsigned char computed_mac[32];
     size_t enc_len, clear_len, i;
     unsigned char pad_len;