Building up "Fixes for SkRWBuffer"

src/core/SkRWBuffer.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAtomics.h"
 #include "SkRWBuffer.h"
 #include "SkStream.h"
@@ skipping 124 matching lines @@
         fHead->validate(fAvailable);
         fHead->unref();
     }
 }
 
 SkROBuffer::Iter::Iter(const SkROBuffer* buffer) {
     this->reset(buffer);
 }
 
 void SkROBuffer::Iter::reset(const SkROBuffer* buffer) {
-    if (buffer) {
+    if (buffer && buffer->fHead) {
         fBlock = &buffer->fHead->fBlock;
         fRemaining = buffer->fAvailable;
     } else {
         fBlock = nullptr;
         fRemaining = 0;
     }
 }
 
 const void* SkROBuffer::Iter::data() const {
     return fRemaining ? fBlock->startData() : nullptr;
@@ skipping 179 matching lines @@
     const SkROBuffer*   fBuffer;
     SkROBuffer::Iter    fIter;
     size_t              fLocalOffset;
     size_t              fGlobalOffset;
 };
 
 SkStreamAsset* SkRWBuffer::newStreamSnapshot() const {
     SkAutoTUnref<SkROBuffer> buffer(this->newRBufferSnapshot());
     return new SkROBufferStreamAsset(buffer);
 }