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

tests/StreamTest.cpp

Index: tests/StreamTest.cpp
diff --git a/tests/StreamTest.cpp b/tests/StreamTest.cpp
index d307f513183e4c520005c3e569cb936eb9a509d9..838a8a4d0f1ed285a81be7aa0d12a9a99e373834 100644
--- a/tests/StreamTest.cpp
+++ b/tests/StreamTest.cpp
@@ -391,5 +391,11 @@ DEF_TEST(StreamCopy, reporter) {
     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());
 }
+