Silence "use of possibly-NULL ‘msgstr’" warnings.

Silence "use of possibly-NULL ‘msgstr’" warnings.

* gettext-tools/src/write-po.c: Include verify.h.
(message_print_comment_filepos): Assume that this xasprintf call returns
non-NULL.
* gettext-tools/src/xg-message.c: Include verify.h.
(remember_a_message, remember_a_message_plural): Assume that this xasprintf call
returns non-NULL.
* gettext-tools/src/xgettext.c: Include verify.h.
(construct_header): Assume that this xasprintf call returns non-NULL.