fix misc asserts and checks found by fuzzer

src/core/SkCanvas.cpp

 /*
  * Copyright 2008 The Android Open Source Project
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmapDevice.h"
 #include "SkCanvas.h"
 #include "SkCanvasPriv.h"
@@ skipping 1054 matching lines @@
     return true;
 #endif
 }
 
 bool SkCanvas::clipRectBounds(const SkRect* bounds, SaveLayerFlags saveLayerFlags,
                               SkIRect* intersection, const SkImageFilter* imageFilter) {
     SkIRect clipBounds;
     if (!this->getClipDeviceBounds(&clipBounds)) {
         return false;
     }
+    SkASSERT(!clipBounds.isEmpty());
 
     const SkMatrix& ctm = fMCRec->fMatrix;  // this->getTotalMatrix()
 
     if (imageFilter) {
-        imageFilter->filterBounds(clipBounds, ctm, &clipBounds);
+        if (!imageFilter->filterBounds(clipBounds, ctm, &clipBounds) || clipBounds.isEmpty()) {
+            return false;
+        }
         if (bounds && !imageFilter->canComputeFastBounds()) {
             bounds = nullptr;
         }
     }
     SkIRect ir;
     if (bounds) {
         SkRect r;
 
         ctm.mapRect(&r, *bounds);
         r.roundOut(&ir);
@@ skipping 681 matching lines @@
 
 bool SkCanvas::getClipDeviceBounds(SkIRect* bounds) const {
     const SkRasterClip& clip = fMCRec->fRasterClip;
     if (clip.isEmpty()) {
         if (bounds) {
             bounds->setEmpty();
         }
         return false;
     }
 
+    SkASSERT(!clip.getBounds().isEmpty());
     if (bounds) {
         *bounds = clip.getBounds();
     }
     return true;
 }
 
 const SkMatrix& SkCanvas::getTotalMatrix() const {
     return fMCRec->fMatrix;
 }
 
@@ skipping 1173 matching lines @@
     }
 
     if (matrix) {
         canvas->concat(*matrix);
     }
 }
 
 SkAutoCanvasMatrixPaint::~SkAutoCanvasMatrixPaint() {
     fCanvas->restoreToCount(fSaveCount);
 }