This fixes a null pointer access in FindSharedFunctionInfo that
was introduced when adding a guard to top level function
compilation.

Bug: chromium:1185540
Change-Id: I24b9752637aba0e660bd8f20be83522e1009b69f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2742194
Commit-Queue: Kim-Anh Tran <kimanh@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73258}