client: make sure the monitor index defaults to -1

This ensures that the menu spawns on the current monitor by default when
using X11 or on all monitors when using Wayland.

client/common/common.c

@@ -278,6 +278,9 @@ do_getopt(struct client *client, int *argc, char **argv[])
      * Either break the interface and make them --sf, --sb (like they are now),
      * or parse them before running getopt.. */
 
+    /* Set default monitor index to -1 (maybe be overwritten with -m below). */
+    client->monitor = -1;
+
     for (optind = 0;;) {
         int32_t opt;