gettext 1.0 changed msginit to merge into existing output files
instead of overwriting them. When po4a passes an empty PO file as the
output target, this produces a PO with stripped headers and corrupted
UTF-8 content (e.g. "nämes" becomes "nmes").
Fix by removing empty output files before calling msginit, so it
creates a fresh PO file with proper headers.
https://github.com/mquinson/po4a/issues/636
