Problem: Item_func_weight_string() could allocate unlimited amount
of memory, which led to server crash.

Fix: honor max_allowed_packet in Item_func_weight_string::val_str()

  @ mysql-test/r/func_weight_string.result
  @ mysql-test/t/func_weight_string.test
  Adding tests

  @ sql/item_strfunc.cc
  Don't return results longer than max_allowed_packet