Initialize b:UltiSnipsSnippetDirectories if it doesn't exist.

Now that UltiSnips pays attention to b:UltiSnipsSnippetDirectories and
our workaround is removed, there's the possibility that
b:UltiSnipsSnippetDirectories doesn't exist.  Let's borrow from our old
behavior by copying the global setting to our buffer-local variable,
and append to the copy.