Ensure that a canvas is always backed by some form of SkDevice.

bug: 8518884
Change-Id: Id6c597179878ce5b938c00422e88bef5f12e2e1f

core/jni/android/graphics/Canvas.cpp

@@ -77,7 +77,14 @@ public:
     }
 
     static SkCanvas* initRaster(JNIEnv* env, jobject, SkBitmap* bitmap) {
-        return bitmap ? new SkCanvas(*bitmap) : new SkCanvas;
+        if (bitmap) {
+            return new SkCanvas(*bitmap);
+        } else {
+            // Create an empty bitmap device to prevent callers from crashing
+            // if they attempt to draw into this canvas.
+            SkBitmap emptyBitmap;
+            return new SkCanvas(emptyBitmap);
+        }
     }
     
     static void copyCanvasState(JNIEnv* env, jobject clazz,