Prevent memory corruption from use-after-free

Bug: 19035637

If an app tries to call recycle() on a Bitmap that has
already been finalized it will result in use-after-frees. This is
bad. Avoid this by setting the pointer to 0 and checking for this

Change-Id: I12d73703a0f95b05fe4c2fd8e9c01b6a3f2f023b