Fix errors in fsharp-ac--can-make-request.
Fix errors in fsharp-ac--can-make-request.

Ensure buffer-file-name is not nil before processing it.

nosami/xamarin-monodevelop-fsharp-addin,nosami/xamarin-monodevelop-fsharp-addin,Ming-Tang/fsharpbinding,fsharp/xamarin-monodevelop-fsharp-addin,Ming-Tang/fsharpbinding,fsharp/fsharpbinding,fsharp/xamarin-monodevelop-fsharp-addin,fsharp/fsharpbinding,Ming-Tang/fsharpbinding,fsharp/fsharpbinding