odt: Restore writing creator metadata.

Commit 4d7a2ed5 "odt: Remove dependency on <pwd.h> because mingw32 does
not have it" stopped writing creator metadata to ODT output files.  This
commit adds it back, but only on systems that have <pwd.h>.

This commit also adds avoids a null pointer dereference if getpwuid()
returns NULL.  That should happen only rarely, but it is best to handle it
properly.

At John Darrington's request.