Fix CompactionPartiallyAbortedPageWithStoreBufferEntries

The test was reading from undefined stack addresses instead of creating
a string filled with characters that look like a new space pointer.

Bug: v8:6953
Change-Id: I2c0a9034076012746bd70325a4f21c63f4c264fa
Reviewed-on: https://chromium-review.googlesource.com/725322Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48668}

test/cctest/heap/test-compaction.cc

@@ -347,7 +347,7 @@ HEAP_TEST(CompactionPartiallyAbortedPageWithStoreBufferEntries) {
       Address broken_address = holder->address() + 2 * kPointerSize + 1;
       // Convert it to a vector to create a string from it.
       Vector<const uint8_t> string_to_broken_addresss(
-          reinterpret_cast<const uint8_t*>(&broken_address), 8);
+          reinterpret_cast<const uint8_t*>(&broken_address), kPointerSize);
 
       Handle<String> string;
       do {