randr: quit when no screens found & not active

fvwm would quit when there were no screens detected by RandR.  However,
this can be problematic with laptop screens which can be turned off.

If fvwm receives an RandR event to rescan screens, that can therefore
cause RandR to tell us there's no screens found.  This is not true
though since the number of *configured* screens from fvwm's perspective
is still the same.

In such cases then, fvwm should only quit when the number of configured
screens is 0 *and* RandR hasn't detected any screens either.

Problem reported by @farhaven (Gregor Best)

libs/FScreen.c

@@ -43,6 +43,7 @@ static int grav_matrix[3][3] =
 
 static Display *disp;
 static bool		 is_randr_present;
+static bool		 randr_initialised;
 
 static void		 scan_screens(Display *);
 static struct monitor	*monitor_by_name(const char *);
@@ -83,6 +84,9 @@ monitor_scan_edges(struct monitor *m)
 {
 	struct monitor	*m_loop;
 
+	if (m == NULL)
+		return;
+
 	m->edge.top = m->edge.bottom = m->edge.left = m->edge.right =
 		MONITOR_OUTSIDE_EDGE;
 
@@ -424,12 +428,12 @@ static void
 scan_screens(Display *dpy)
 {
 	XRRMonitorInfo		*rrm;
-	struct monitor 		*m;
+	struct monitor 		*m = NULL;
     	int			 i, n = 0;
 	Window			 root = RootWindow(dpy, DefaultScreen(dpy));
 
 	rrm = XRRGetMonitors(dpy, root, false, &n);
-	if (n <= 0) {
+	if (n <= 0 && (!randr_initialised && monitor_get_count() == 0)) {
 		fvwm_debug(__func__, "get monitors failed\n");
 		exit(101);
 	}
@@ -492,9 +496,13 @@ void FScreenInit(Display *dpy)
 	struct monitor		*m;
 	int			 err_base = 0, major, minor;
 
+	if (randr_initialised)
+		return;
+
 	disp = dpy;
 	randr_event = 0;
 	is_randr_present = false;
+	randr_initialised = false;
 
 	if (TAILQ_EMPTY(&monitor_q))
 		TAILQ_INIT(&monitor_q);
@@ -532,6 +540,7 @@ void FScreenInit(Display *dpy)
 	XRRFreeScreenResources(res);
 
 	scan_screens(dpy);
+	randr_initialised = true;
 	is_tracking_shared = false;
 
 	TAILQ_FOREACH(m, &monitor_q, entry) {