Bug 998821. Only use XShm for drawing if XShm allocation actually succeeded. r=karlt

--HG--
extra : rebase_source : 9d0fc1b9c7b02c53674b4593eaf1cd703587a469

widget/gtk/nsWindow.cpp

@@ -2220,7 +2220,7 @@ nsWindow::OnExposeEvent(cairo_t *cr)
         }
     }
 #  ifdef MOZ_HAVE_SHMIMAGE
-    if (nsShmImage::UseShm() && MOZ_LIKELY(!mIsDestroyed)) {
+    if (mShmImage && MOZ_LIKELY(!mIsDestroyed)) {
 #if (MOZ_WIDGET_GTK == 2)
         mShmImage->Put(mGdkWindow, exposeRegion.mRects, exposeRegion.mRectsEnd);
 #else