AIX returns NULL rather than a valid heap address.

TEST=cctest/test-run-wasm-module/Run_WasmModule_CallAdd_rev
R=titzer@chromium.org, jochen@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/1934633002
Cr-Commit-Position: refs/heads/master@{#35913}