Move pointer validation to be under an ifdef intended to be used only when Halide cuda internals are being modified.
Move pointer validation to be under an ifdef intended to be used only
when Halide cuda internals are being modified.

psuriana/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide,psuriana/Halide