Fix memleak again

Memleak was reinroduced in d92c576aa5
It was already fixed in ac5ce105ac

But the rebase peetahs rebase ontop of master took the wrong changes.

I decided to pull anyways and fix since reviewing/giving feedback
without GH probably takes longer.

src/command/cmd_funcs.c

@@ -8921,10 +8921,14 @@ cmd_urlopen(ProfWin *window, const char *const command, gchar **args)
             return TRUE;
         }
 
-        gchar *argv[] = {prefs_get_string(PREF_URL_OPEN_CMD), args[0], NULL};
+        gchar* cmd = prefs_get_string(PREF_URL_OPEN_CMD);
+        gchar *argv[] = {cmd, args[0], NULL};
+
         if (!call_external(argv, NULL, NULL)) {
           cons_show_error("Unable to open url: check the logs for more information.");
         }
+
+        g_free(cmd);
     } else {
         cons_show("urlopen not supported in this window");
     }