Fixed potetial segmentation fault for xwin_should_render, addresses #19

shaders/rc.glsl

@@ -40,7 +40,7 @@
               Has very little performance impact.
     
    "none"   - Disable window opacity completely. */
-#request setopacity "xroot"
+#request setopacity "native"
 
 /* OpenGL context and GLSL shader versions, do not change unless
    you *absolutely* know what you are doing. */

xwin.c

@@ -77,6 +77,9 @@ bool xwin_should_render(struct renderer* rd) {
         goto close; /* if an error occurs here, the WM probably isn't EWMH compliant */
     }
 
+    if (!nitems)
+        goto close;
+    
     Window active = ((Window*) data)[0];
 
     prop = XInternAtom(d, "_NET_WM_STATE", true);