In strict mode a timestamp(0) column could be directly assigned from
another timestamp(N>0) column with the value '1970-01-01 00:00:00.1'
(at time zone '+00:00'), or with any other value '1970-01-01 00:00:00.XXXXXX'
with non-zero microsecond value XXXXXX.
This assignment happened silently without warnings or errors.

It worked as follows:

- The value {tv_sec=0, tv_usec=100000}, which is '1970-01-01 00:00:00.1'
  was rounded to {tv_sec=0, tv_usec=0}, which is '1970-01-01 00:00:00.0'

- Then {tv_sec=0, tv_usec=0} was silently re-interpreted as zero datetime.

After the fix this assignment always raises a warning,
which in case of the strict mode is escalated to an error.

The problem in this scenario is that '1970-01-01 00:00:00' cannot be stored,
because its timeval value {tv_sec=0, tv_usec=0} is reserved for zero datetimes.
Thus the warning should be raised no matter if sql_mode allows or disallows
zero dates.