parameters": when unpacking binary time recieved from client, handle
the case when length is 0: it means all MYSQL_TIME members are zero.


include/my_time.h:
  Declaration for set_zero_time: a tiny piece of code, which I
  see no reason to not reuse.
libmysql/libmysql.c:
  set_zero_time implementation is now shared between client and
  server.
sql-common/my_time.c:
  set_zero_time implementation added.
sql/sql_prepare.cc:
  A fix for Bug#4231 "Wrong result with MYSQL_TIME parameters": 
  when unpacking binary time recieved from client, handle the
  case when length is 0: it means all MYSQL_TIME members are zero.
tests/client_test.c:
  Test case for bug#4231 "Wrong result with MYSQL_TIME parameters"