magit-run-git-gui-blame: fix when linenum is missing

2eb34da broke magit-run-git-gui-blame by erroneously changing the
argument to magit-file-relative-name to be a directory.  This prevented
the linenum argument from being properly populated.
Merge pull request #1530 from aspiers/fix-git-gui-blame

magit-run-git-gui-blame: fix when linenum is missing