Do not use multi-threads with -Ff (custom filter). It's broken.

src/grdfilter.c

@@ -895,7 +895,15 @@ static int parse (struct GMT_CTRL *GMT, struct GRDFILTER_CTRL *Ctrl, struct GMT_
 
 #ifdef HAVE_GLIB_GTHREAD
 	/* Make the default equal to the OMP case where we use all threads if not stated otherwise. */
-	if (!GMT->common.x.active) GMT->common.x.n_threads = gmtlib_get_num_processors();
+	if (Ctrl->F.custom && GMT->common.x.n_threads > 1) {
+		GMT->common.x.n_threads = 1;
+		GMT->common.x.active = false;
+		GMT_Report (GMT->parent, GMT_MSG_WARNING, "Sorry, custom filtering does not support multiple threads. Reset to 1.\n" );
+	}
+	if (!GMT->common.x.active) {
+		GMT->common.x.n_threads = gmtlib_get_num_processors();
+		GMT_Report (GMT->parent, GMT_MSG_INFORMATION, "Enable all available threads (up to %d)\n", gmtlib_get_num_processors());
+	}
 #endif
 
 	n_errors += gmt_M_check_condition (GMT, !Ctrl->G.active, "Option -G: Must specify output file\n");