Nullptr deref in the wasm async fuzzer.

The following revision refers to this bug:
  https://chromium.googlesource.com/v8/v8.git/+/3ced15cb037dabb05139b8c069aeadc858194ec1

commit 3ced15cb037dabb05139b8c069aeadc858194ec1
Author: Clemens Hammacher <clemensh@chromium.org>
Date: Fri Sep 08 19:12:27 2017

[wasm] [fuzzer] Fix segfault

Even though we were generating additional arguments with default value
in the case that the caller was not providing enough, we then passed
the original pointer, leading to potential out-of-bounds accesses.

R=ahaas@chromium.org

Bug:  chromium:763294 , chromium:763297 
Change-Id: Id18622d0d40e0408e26a5fc6f97494b5f9e18d17
Reviewed-on: https://chromium-review.googlesource.com/657699
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47930}
[modify] https://crrev.com/3ced15cb037dabb05139b8c069aeadc858194ec1/test/common/wasm/wasm-module-runner.cc

ClusterFuzz has detected this issue as fixed in range 500678:500748.

Detailed report: https://clusterfuzz.com/testcase?key=5599349005615104

Fuzzer: libFuzzer_v8_wasm_async_fuzzer
Job Type: libfuzzer_chrome_asan_debug
Platform Id: linux

Crash Type: Null-dereference READ
Crash Address: 0x000000000000
Crash State:
  type
  Push
  InitFrame
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan_debug&range=500422:500485
Fixed: https://clusterfuzz.com/revisions?job=libfuzzer_chrome_asan_debug&range=500678:500748

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5599349005615104

See https://chromium.googlesource.com/chromium/src/+/master/testing/libfuzzer/reproducing.md for more information.

If you suspect that the result above is incorrect, try re-doing that job on the test case report page.