Fix tagbar bug

I filed a bug report upstream, but it appears that the maintainer has been down and out for a few months.  I don't believe he's going to get to it any time soon.  In the meantime, let's patch the bug here.

You might enjoy knowing that I discovered this when editing a reStructuredText file, and I hit a line and saw `die` appear in the status line.  It turns out the bug caused it to look up tags on the file I was just editing, which happened to be a script.  And the line I was on would have been in the `die` function in the script.  I thought Vim was trying to tell me something for a minute. :-)