gssapi: use gss_buffer_desc consistently

Problem: one call to gss_import_name() includes the terminating
NULL in a gss_buffer_desc.length, and one doesn't.

According to the examples at:
http://docs.oracle.com/cd/E19253-01/816-4863/overview-22/index.html
the NULL should be included in the length.

Solution:  Fix one case to include the terminating NULL in the length.

src/gssapi_client.cpp

@@ -173,7 +173,7 @@ int zmq::gssapi_client_t::initialize_context ()
     // First time through, import service_name into target_name
     if (target_name == GSS_C_NO_NAME) {
         send_tok.value = service_name;
-        send_tok.length = strlen(service_name);
+        send_tok.length = strlen(service_name) + 1;
         OM_uint32 maj = gss_import_name(&min_stat, &send_tok,
                                         GSS_C_NT_HOSTBASED_SERVICE,
                                         &target_name);