X11: check crtcInfo for NULL when polling monitors

src/x11_monitor.c

@@ -151,6 +151,11 @@ void _glfwPollMonitorsX11(void)
             }
 
             XRRCrtcInfo* ci = XRRGetCrtcInfo(_glfw.x11.display, sr, oi->crtc);
+            if (!ci) {
+                XRRFreeOutputInfo(oi);
+                continue;
+            }
+
             if (ci->rotation == RR_Rotate_90 || ci->rotation == RR_Rotate_270)
             {
                 widthMM  = oi->mm_height;