In some (rare) cases, the context depth passed to a dynamic variable lookup can
be zero. In these cases, the fast path for the lookup (i.e. load from context or
global) can always be taken, as there is no need to search the current context.

However, with no slow path checks, the bytecode graph builder had a null
environment for the slow path, causing segfaults when this graph was built.

This patch adds a null check for the slow path environment, and skips building
the slow path if the environment is null.

BUG=chromium:652186

Review-Url: https://codereview.chromium.org/2385123002
Cr-Commit-Position: refs/heads/master@{#39949}