Fix a crash in pring spooler.

If the printer capabilities are reported with a delay which is pretty
common we were trying to evaluate the preview page dimensions without
having a media size resulting in a NPE.

bug:17501953

Change-Id: I6765137f61b8fd060e9eabff07e632875d934963

packages/PrintSpooler/src/com/android/printspooler/ui/PageAdapter.java

@@ -384,6 +384,10 @@ public final class PageAdapter extends Adapter implements
     }
 
     private void updatePreviewAreaPageSizeAndEmptyState() {
+        if (mMediaSize == null) {
+            return;
+        }
+
         final int availableWidth = mPreviewArea.getWidth();
         final int availableHeight = mPreviewArea.getHeight();