Fix NPE in GradientDrawable constructor

The mColors member can be null, as can be seen in
GradientDrawable.setSolidColor() or the plain constructor. In that case, an NPE
will be thrown on attempts to derive a new GradientDrawable from the instance
using the private constructor GradientDrawable(GradientDrawable).

The problem also occurs when inflating a GradientDrawable from XML without
defining the start, center and end colors and then calling mutate() on the
instance.