Fix FastSharedBufferReaderTest::readAllOverlappingLastSegmentBoundary

GetConsecutiveData may return pointer into internal cache.
However the test always check buffer which accidentally contains the same data
from the previous iteration.

Found compiling with -ftrivial-auto-var-init=pattern

Change-Id: Ib72585fd598d23bac6bcb976d7b3f309536a8376
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1769232
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Auto-Submit: Vitaly Buka <vitalybuka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690100}

third_party/blink/renderer/platform/image-decoders/fast_shared_buffer_reader_test.cc

@@ -145,9 +145,9 @@ TEST(FastSharedBufferReaderTest, readAllOverlappingLastSegmentBoundary) {
   SegmentReaders reader_struct(data);
   for (auto segment_reader : reader_struct.segment_readers) {
     FastSharedBufferReader reader(segment_reader);
-    char buffer[kDataSize];
-    reader.GetConsecutiveData(0, kDataSize, buffer);
-    ASSERT_FALSE(memcmp(buffer, reference_data, kDataSize));
+    char buffer[kDataSize] = {};
+    const char* result = reader.GetConsecutiveData(0, kDataSize, buffer);
+    ASSERT_FALSE(memcmp(result, reference_data, kDataSize));
   }
 }