Wrong identifier used to check Encrypt-then-MAC flag

This commit fixes a comparison of ssl_session->encrypt_then_mac against the
ETM-unrelated constant SSL_EXTENDED_MS_DISABLED. Instead, SSL_ETM_DISABLED
should be used.

The typo is has no functional effect since both constants have the same value 0.
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index 0f0369a..d141b05 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -1963,7 +1963,7 @@
     const ssl_ciphersuite_t *suite = NULL;
     const cipher_info_t *cipher = NULL;
 
-    if( ssl->session_negotiate->encrypt_then_mac == SSL_EXTENDED_MS_DISABLED ||
+    if( ssl->session_negotiate->encrypt_then_mac == SSL_ETM_DISABLED ||
         ssl->minor_ver == SSL_MINOR_VERSION_0 )
     {
         *olen = 0;