Previously, UDF *_init functions were passed constant strings with erroneous lengths.
The length came from the containing variable's size, not the length of the value itself.

Now the *_init functions get the constant as a null terminated string with the correct
length supplied.


mysql-test/r/udf.result:
  Test case to check constants passed UDFs.
mysql-test/t/udf.test:
  Test case to check constants passed UDFs.
sql/item_func.cc:
  UDF _init functions are now passed the length of the constants, rather than the max
  length of the var containing the constant.
sql/udf_example.c:
  Added check_const_len functions. The check_const_len_init functions checks that the
  lengths of constants are correctly passed.