ui: check g_malloc return (found by clang scan).

Change-Id: I96df8a87f5f213fc1b77c485899cf373987413ac
Reviewed-on: https://code.wireshark.org/review/25252
Petri-Dish: Dario Lombardo <lomato@gmail.com>
Tested-by: Petri Dish Buildbot
Reviewed-by: Anders Broman <a.broman58@gmail.com>

ui/util.c

@@ -53,6 +53,9 @@ get_args_as_string(int argc, char **argv, int optindex)
     int i;
     char *argstring;
 
+    /* We don't allow a null-list call */
+    g_assert(argc - optindex > 0);
+
     /*
      * Find out how long the string will be.
      */