g_date_time_new, and g_date_time_new_utc return NULL if inputs are out
of range, and currently mm_new_iso8601_time passes the GDateTime created
by those two functions to date_time_format_iso8601 without checking for
NULL values, causing a g_date_time_format_iso8601 crash if PDU data is
corrupted with wrong date.
To prevent this, mm_new_iso8601_time now can return NULL and set a new
GError if GDateTime created by g_date_time_new is NULL.
Fixes#546
ac243f9467