initPanFrames: guard mapped panframes

initPanFrames() can be called from a few different commands to ensure
panframes are mapped.  Since there's no way of knowing which command
will initiate the panframes, guard against this so we don't end up
unmapping the panframes.

Fixes #543

fvwm/virtual.c

@@ -109,6 +109,7 @@
  */
 static int edge_thickness = 2;
 static int last_edge_thickness = 2;
+static bool pan_frames_mapped = false;
 
 static void store_desktop_cmd(int, char *);
 static int number_of_desktops(struct monitor *);
@@ -1288,6 +1289,9 @@ void initPanFrames(void)
 	int saved_thickness;
 	struct monitor	*m;
 
+	if (pan_frames_mapped)
+		return;
+
 	/* Not creating the frames disables all subsequent behavior */
 	/* TKP. This is bad, it will cause an XMap request on a null window
 	 * later*/
@@ -1340,6 +1344,7 @@ void initPanFrames(void)
 		checkPanFrames(m);
 	}
 	edge_thickness = saved_thickness;
+	pan_frames_mapped = true;
 	fvwm_debug(__func__, "finished setting up per-monitor panframes");
 }