lib: clamp return value of g_mime_utils_header_decode_date to >=0

lib: clamp return value of g_mime_utils_header_decode_date to >=0

For reasons not completely understood at this time, gmime (as of
2.6.22) is returning a date before 1900 on bad date input. Since this
confuses some other software, we clamp such dates to 0,
i.e. 1970-01-01.