glDeleteSync can only delete an existing FenceSync object.

Change-Id: Ieb78894175235cc97c67ff0324d59bb5d7619918
Reviewed-on: https://swiftshader-review.googlesource.com/13829Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>