Bug#42662: maketime() and signedness

Item_time_typecast::val_int() dropped sign from
MYSQL_TIME gotten using from get_time().

Propagates sign now.

mysql-test/r/func_sapdb.result:
  Show that comparisons with TIME(-...) work now.
mysql-test/t/func_sapdb.test:
  Show that comparisons with TIME(-...) work now.
sql/item_timefunc.cc:
  don't drop sign when converting from MYSQL_TIME