Avoid SIGSEGV in Bitmap_writeToParcel.

SkBitmap::getPixels() can return NULL. The rest of the JNI Bitmap
code treats this NULL as if the SkBitmap has transparent black
pixels. Bitmap_writeToParcel now does the same.

Change-Id: I5e70b42b3d22a8aea898ce342e590000325bd0f9