Problem: check missed if the second updatexml() argument is valid.

Fix: check it.


mysql-test/r/xml.result:
  Fix for bug #31438: updatexml still crashes
    - test result.
mysql-test/t/xml.test:
  Fix for bug #31438: updatexml still crashes
    - test case.
sql/item_xmlfunc.cc:
  Fix for bug #31438: updatexml still crashes
    - check if the second argument passed to the updatexml() is valid.