Make SkRWBuffer destruct safely

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 "SkRWBuffer.h"
 #include "SkStream.h"
 
@@ skipping 150 matching lines @@
         fRemaining -= this->size();
         fBlock = fBlock->fNext;
     }
     return fRemaining != 0;
 }
 
 SkRWBuffer::SkRWBuffer(size_t initialCapacity) : fHead(nullptr), fTail(nullptr), fTotalUsed(0) {}
 
 SkRWBuffer::~SkRWBuffer() {
     this->validate();
-    fHead->unref();
+    if (fHead) {
+        fHead->unref();
+    }
 }
 
 void SkRWBuffer::append(const void* src, size_t length) {
     this->validate();
     if (0 == length) {
         return;
     }
 
     fTotalUsed += length;
 
@@ skipping 161 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);
 }