treewide: use helper dixGetScreenPtr() for retrieving ScreenPtr's

Instead of directly accessing the global screenInfo.screens[] array,
let everybody go through a little inline helper. This one also checks
for array bounds - if the screen doesn't exist, return NULL.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

dix/screenint_priv.h

@@ -6,6 +6,7 @@
 #ifndef _XSERVER_DIX_SCREENINT_PRIV_H
 #define _XSERVER_DIX_SCREENINT_PRIV_H
 
+#include <stdbool.h>
 #include <X11/Xdefs.h>
 
 #include "include/callback.h"
@@ -31,6 +32,30 @@ static inline ScreenPtr dixGetMasterScreen(void) {
     return screenInfo.screens[0];
 }
 
+/*
+ * retrieve pointer to screen by it's index. If index is above the total
+ * number of screens, returns NULL
+ *
+ * @param idx screen index
+ * @return pointer to idx'th screen or NULL
+ */
+static inline ScreenPtr dixGetScreenPtr(unsigned int idx) {
+    if (idx < screenInfo.numScreens)
+        return screenInfo.screens[idx];
+    return NULL;
+}
+
+/*
+ * check whether screen with given index exists
+ *
+ * @param idx screen index
+ * @return TRUE if the screen at this index exists
+ */
+static inline bool dixScreenExists(unsigned int idx) {
+    return ((idx < screenInfo.numScreens) &&
+            (screenInfo.screens[idx] != NULL));
+}
+
 /*
  * macro for looping over all screens (up to `screenInfo.numScreens`).
  * Makes a new scopes and declares `walkScreenIdx` as the current screen's