When storing '0001-01-01 10:20:30x', execution went throw the last code
branch in Field_time::store_TIME_with_warning(), around the test
for (ltime->year || ltime->month). This then resulted into wrong results
because:

1. Field_time::store_TIME() does not check YYYYMM against zero.
  It assumes that ltime->days and ltime->hours are already properly set.
  So it mixed days to hours, even when YYYYMM was not zero.

2. Field_time_hires::store_TIME() does not check YYYYMM against zero.
  It assumes that ltime->year, ltime->month, ltime->days and ltime->hours
  are already properly set. So it always mixed days and even months(!) and years(!)
  to hours, using pack_time(). This gave even worse results comparing to #2.

3. Field_timef::store_TIME() did not check the entire YYYYMM for being zero.
  It only checked MM, but did not check YYYY. In case of a zero MM,
  it mixed days to hours, even if YYYY was not zero.
  The wrong code was in TIME_to_longlong_time_packed().

In the new reduction Field_time::store_TIME_with_warning() is responsible
to prepare the YYYYYMMDD part properly in all code branches
(with trailing garbage like 'x' and without trailing garbage).
It was reorganized into a more straightforward style.

Field_time:store_TIME(), Field_time_hires::store_TIME() and
TIME_to_longlong_time_packed() were fixed to do a DBUG_ASSERT
on non-zero ltime->year or ltime->month. The code testing ltime->month
was removed from TIME_to_longlong_time_packed(), as it's now
properly done on the caller level.

Truncation was moved from Field_timef::store_TIME() to
Field_time::store_TIME_with_warning().

So now all thee methods Field_time*::store_TIME() assume a properly
set input value:
- Only zero ltime->year and ltime->month are allowed.
- The value must be already properly truncated according to decimals()
  (this will help to add rounding soon, see MDEV-8894)

A "const" qualifier was added to the argument of Field_time*::store_TIME().