Don't produce data truncation warnings from within cp_buffer_from_ref(). This function
is only used to make index search tuples and data truncation that occurs here has no
relation with truncated values being saved into tables.


mysql-test/r/update.result:
  Testcase for BUG#9103
mysql-test/t/update.test:
  Testcase for BUG#9103
sql/opt_range.cc:
  cp_buffer_from_ref now has THD* parameter
sql/sql_select.h:
  cp_buffer_from_ref now has THD* parameter