{NativeModule::GetCode} can actually return {nullptr} if no code was
compiled yet for a function, e.g. in asm.js where we use lazy
compilation. In that case, we must not try to increment the ref count
on the nonexisting code object.

We had a few errors recently that were hard to reproduce because we do
not have a flag to enable code logging. Clusterfuzz managed to
accomplish this by passing --trace-ic.
In order to test bugs in code logging properly, this CL introduces a
new runtime function called "EnableCodeLoggingForTesting". It registers
a noop {CodeEventListener} and enables code logging in the wasm engine.
We should whitelist this flag in ClusterFuzz to potentially flush out
more bugs.

R=mstarzinger@chromium.org
CC=frgossen@chromium.org

Bug: v8:8217, chromium:961129, chromium:961245, chromium:961128
Change-Id: I2f97c109db70b41531d58580b71f6781beeb8dcb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1602700
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61404}