Explicitly verify state is non null in Drawables.

Related to #233.

library/src/main/java/com/bumptech/glide/load/resource/bitmap/GlideBitmapDrawable.java

@@ -29,6 +29,10 @@ public class GlideBitmapDrawable extends GlideDrawable {
     }
 
     GlideBitmapDrawable(Resources res, BitmapState state) {
+        if (state == null) {
+            throw new NullPointerException("BitmapState must not be null");
+        }
+
         this.state = state;
         final int targetDensity;
         if (res != null) {

library/src/main/java/com/bumptech/glide/load/resource/gif/GifDrawable.java

@@ -75,6 +75,10 @@ public class GifDrawable extends GlideDrawable implements GifFrameManager.FrameC
     }
 
     GifDrawable(GifState state) {
+        if (state == null) {
+            throw new NullPointerException("GifState must not be null");
+        }
+
         this.state = state;
         this.decoder = new GifDecoder(state.bitmapProvider);
         decoder.setData(state.gifHeader, state.data);