Avoid extraneous NUL bytes in .mo files.
Reported by Jakub Wilk <jwilk@jwilk.net> via Santiago Vila <sanvila@unex.es>
in <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=872869>.
* gettext-tools/src/msgl-header.c (msgdomain_list_set_header_field,
message_list_delete_header_field): Adjust msgstr_len when modifying msgstr.
