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

tests/StreamTest.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Resources.h"
 #include "SkData.h"
 #include "SkFrontBufferedStream.h"
@@ skipping 373 matching lines @@
     static const int N = 10000;
     SkAutoTMalloc<uint8_t> src((size_t)N);
     for (int j = 0; j < N; ++j) {
         src[j] = random.nextU() & 0xff;
     }
     // SkStreamCopy had two code paths; this test both.
     DumbStream dumbStream(src.get(), (size_t)N);
     stream_copy_test(reporter, src, N, &dumbStream);
     SkMemoryStream smartStream(src.get(), (size_t)N);
     stream_copy_test(reporter, src, N, &smartStream);
+}
 
+DEF_TEST(StreamEmptyStreamMemoryBase, r) {
+    SkDynamicMemoryWStream tmp;
+    SkAutoTDelete<SkStreamAsset> asset(tmp.detachAsStream());
+    REPORTER_ASSERT(r, nullptr == asset->getMemoryBase());
 }
+