Don't throw an exception if 'last' doesn't exist

The issue was caused by a change in behaviour introduced in ultim's
commits (see <http://forum.doom9.org/showthread.php?p=1643908#post1643908>):
previously a variable called 'last' was always defined (but maybe left
initialized).
Prefer VSImport over AVISource when evaluating vpy files

<https://github.com/chikuzen/VapourSource/>

Allows outputing >8-bit color spaces, using stacked format
[avisynth] Correct boolean AVS_Value
Escape fnmatch patterns

See <http://bugs.python.org/issue8402>
Don't assume that $InternalFunctions$ exists

Fix for current avisynth+ git