SkStream: don't segfault on empty asset getMemoryBase().

src/core/SkStream.cpp

 
 /*
  * Copyright 2006 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 "SkStream.h"
@@ skipping 777 matching lines @@
         that->fOffset = this->fOffset;
         that->fCurrentOffset = this->fCurrentOffset;
         return that.detach();
     }
 
     size_t getLength() const override {
         return fSize;
     }
 
     const void* getMemoryBase() override {
-        if (nullptr == fBlockMemory->fHead->fNext) {
+        if (nullptr != fBlockMemory->fHead &&
+            nullptr == fBlockMemory->fHead->fNext) {
             return fBlockMemory->fHead->start();
         }
         return nullptr;
     }
 
 private:
     SkAutoTUnref<SkBlockMemoryRefCnt> const fBlockMemory;
     SkDynamicMemoryWStream::Block const * fCurrent;
     size_t const fSize;
     size_t fOffset;
@@ skipping 157 matching lines @@
     while (true) {
         count = input->read(scratch, sizeof(scratch));
         if (0 == count) {
             return true;
         }
         if (!out->write(scratch, count)) {
             return false;
         }
     }
 }