This patch fixed segfault when supplying multiple options like '-f' for iftop.
The fixing has been integrated into open iftop-1.0 pre-release version. We can
see the fixing log here: 
http://www.ex-parrot.com/~pdw/iftop/cvs/iftop/ChangeLog

--- cfgfile.c.orig	2013-12-02 00:37:09.851645000 -0800
+++ cfgfile.c	2013-12-02 00:31:53.734822000 -0800
@@ -230,8 +230,13 @@
     stringmap S;
 
     S = stringmap_find(config, directive);
-    if (S) stringmap_delete_free(S);
-    stringmap_insert(config, directive, item_ptr(xstrdup(s)));
+    if (S) {
+	xfree(S->d.v);
+	S->d = item_ptr(xstrdup(s));
+    }
+    else {
+	stringmap_insert(config, directive, item_ptr(xstrdup(s)));
+    }
 }
 
 int read_config(char *file, int whinge_on_error) {