Fix NullPointerException in NinePatch constructor

NinePatch.mPaint may be null and most methods in this class handle
that case properly. However, the constructor which derives a new
NinePatch from an existing instance assumes that mPaint is non-null.
This results in an unexpected NullPointerException, for example when
attempting to call NinePatchDrawable.mutate() on an instance that was
created from a resource.

Small unrelated fix in same file: Remove unused private mRect member.