Fix a typo introduced in the first series implementing
'set-session-server-name!' whereby the trailing nul character would
be passed on the wire, thereby breaking connections.

diff --git a/guile/src/core.c b/guile/src/core.c
index 82fd573..2778205 100644
--- a/guile/src/core.c
+++ b/guile/src/core.c
@@ -719,7 +719,7 @@ SCM_DEFINE (scm_gnutls_set_session_server_name_x, "set-session-server-name!",
   c_name = scm_to_locale_string (name);
 
   err = gnutls_server_name_set (c_session, c_type, c_name,
-				strlen (c_name) + 1);
+				strlen (c_name));
   free (c_name);
 
   if (EXPECT_FALSE (err != GNUTLS_E_SUCCESS))