crashes server
 Check for null value is reliable only after calling some of the 
 val_xxx() methods. If the val_xxx() method is not called
 the null_value flag will be set only for certain types of NULL
 values (like SQL constant NULLs for example).
 This caused a crash while trying to dereference a NULL pointer
 that is returned by val_str() for NULL values.
 Fixed by swapping the order of val_xxx() and null_value check.


mysql-test/r/udf.result:
  Bug #25382: Passing NULL to an UDF called from stored procedures 
   crashes server
   - test case
mysql-test/t/udf.test:
  Bug #25382: Passing NULL to an UDF called from stored procedures 
   crashes server
   - test case
sql/item_func.cc:
  Bug #25382: Passing NULL to an UDF called from stored procedures 
   crashes server
   - reliably check null_value