"ProcessState contains information about an exited process, available after a call to Wait or Run"

As ProcessState is only assigned after `Wait()` has completed, and this is run on another goroutine, it may be nil.
Check before using it.

Change-Id: I319f0703b3547186b53caca8d9d70bc5cd738d34
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43993Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Ben Clayton <bclayton@google.com>