The is_null value was initialized once and thereafter only set to indicate
NULL, and never unset to indicate not-NULL.

Now set is_null to false, in addition to only setting it to true when the value 
in question is null.


mysql-test/r/udf.result:
  Add result.
mysql-test/t/udf.test:
  Add test.
sql/sql_udf.h:
  Initialize is_null to false before trying to use it, so that historical NULLs
  don't affect our operation.