glibc declares write() with the warn_unused_result attribute, so we need
to check the return value to avoid a compiler warning.

(This warning only seems to occur when gcc optimizations are enabled)

Change-Id: I31962aae63d0a12eecfe44bb7920a68b29c05d8a
Reviewed-on: https://gerrit.libreoffice.org/13927Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>