timedatectl: assert timezone is not null in setenv() call.

timedatectl: assert timezone is not null in setenv() call.

setenv is declared as:

extern int setenv (const char *__name, const char *__value, int __replace)
     __THROW __nonnull ((2));

And i->timezone can be NULL, if for example /etc/localtime is
missing. Previously that worked, but now result in a libc dumping
core, as seen with gcc 2.22, due to:

https://sourceware.org/ml/glibc-cvs/2015-q2/msg00075.html