check for null-device pixels in onWritePixels, so we don't crash

src/core/SkBitmapDevice.cpp

Index: src/core/SkBitmapDevice.cpp
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index 8524f0b72b4312746d10a307767bda14b6e7acb8..724f17fe51c8c53c7e509c982d2dfc22220f3b2a 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -352,6 +352,11 @@ static bool write_pixels(const SkImageInfo& dstInfo, void* dstPixels, size_t dst
 
 bool SkBitmapDevice::onWritePixels(const SkImageInfo& srcInfo, const void* srcPixels,
                                    size_t srcRowBytes, int x, int y) {
+    // since we don't stop creating un-pixeled devices yet, check for no pixels here
+    if (NULL == fBitmap.getPixels()) {
+        return false;
+    }
+
     SkImageInfo dstInfo = fBitmap.info();
     dstInfo.fWidth = srcInfo.width();
     dstInfo.fHeight = srcInfo.height();