Gracefully handle invalid PO(T) files with duplicate header.

If a file has more than one msgid "" entry without msgctxt (i.e.
header), Poedit would try to parse subsequent occurences as headers.
But they'd typically be empty, resulting in empty header and e.g.  wrong
charset info in Poedit.

Ignore (and discard) any headers other than the first one.  It's too
minor an issue to throw an error about it.