Fix dwarf debug line info crash

[Nathan Sidwell]

Hi,
I met a compiler that emits dwarf line sections with an initially empty
filename table. One of the first things it does is to define a filename
entry.

Fixed thusly, ok?

nathan
-- 
Nathan Sidwell    ::   http://www.codesourcery.com   ::     CodeSourcery LLC
          The voices in my head said this was stupid too
nathan@codesourcery.com    ::     http://www.planetfall.pwp.blueyonder.co.uk

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