fix a crasher in surfaceflinger

this would happen if being told to draw before a buffer
was available.

Change-Id: I46d121c73e883078cdbf952063e38b0076f79038
diff --git a/services/surfaceflinger/Layer.cpp b/services/surfaceflinger/Layer.cpp
index 32f300f..383c045 100644
--- a/services/surfaceflinger/Layer.cpp
+++ b/services/surfaceflinger/Layer.cpp
@@ -227,8 +227,13 @@
     } else {
         hwcl->sourceCrop.left   = 0;
         hwcl->sourceCrop.top    = 0;
-        hwcl->sourceCrop.right  = buffer->width;
-        hwcl->sourceCrop.bottom = buffer->height;
+        if (buffer != NULL) {
+            hwcl->sourceCrop.right  = buffer->width;
+            hwcl->sourceCrop.bottom = buffer->height;
+        } else {
+            hwcl->sourceCrop.right  = mTransformedBounds.width();
+            hwcl->sourceCrop.bottom = mTransformedBounds.height();
+        }
     }
 }