The code in Field_timestamp::save_in_field() did not catch
zero datetime and stored it to the other field like a usual value
using store_timestamp_dec(), which knows nothing about zero date and
treats {tv_sec=0, tv_usec=0} as a normal timeval value corresponding to
'1970-01-01 00:00:00 +00:00'.

Fixing the code to catch the special combination (ts==0 && sec_pat==0) and
store it using store_time_dec() with a zero datetime passed as an argument.