X11: Fix queries crashing if monitor disconnected

Merged from downstream kovidgoyal/glfw.  First of many.

Related to #1602.

src/x11_monitor.c

@@ -322,12 +322,16 @@ void _glfwPlatformGetMonitorPos(_GLFWmonitor* monitor, int* xpos, int* ypos)
             XRRGetScreenResourcesCurrent(_glfw.x11.display, _glfw.x11.root);
         XRRCrtcInfo* ci = XRRGetCrtcInfo(_glfw.x11.display, sr, monitor->x11.crtc);
 
-        if (xpos)
-            *xpos = ci->x;
-        if (ypos)
-            *ypos = ci->y;
+        if (ci)
+        {
+            if (xpos)
+                *xpos = ci->x;
+            if (ypos)
+                *ypos = ci->y;
+
+            XRRFreeCrtcInfo(ci);
+        }
 
-        XRRFreeCrtcInfo(ci);
         XRRFreeScreenResources(sr);
     }
 }
@@ -493,9 +497,15 @@ void _glfwPlatformGetVideoMode(_GLFWmonitor* monitor, GLFWvidmode* mode)
             XRRGetScreenResourcesCurrent(_glfw.x11.display, _glfw.x11.root);
         XRRCrtcInfo* ci = XRRGetCrtcInfo(_glfw.x11.display, sr, monitor->x11.crtc);
 
-        *mode = vidmodeFromModeInfo(getModeInfo(sr, ci->mode), ci);
+        if (ci)
+        {
+            const XRRModeInfo* mi = getModeInfo(sr, ci->mode);
+            if (mi)  // mi can be NULL if the monitor has been disconnected
+                *mode = vidmodeFromModeInfo(mi, ci);
+
+            XRRFreeCrtcInfo(ci);
+        }
 
-        XRRFreeCrtcInfo(ci);
         XRRFreeScreenResources(sr);
     }
     else