fragment is not well-formed xml

Problem:
- ExtractValue silently returned NULL if a wrong XML value is passed.
- In some cases "unexpected END-OF-INPUT" error was not detected, and
  a non-NULL result could be returned for a bad XML value.

Fix:
- Adding warning messages, to make user aware why NULL was returned.
- Missing "unexpected END-OF-INPUT" error is reported now.


mysql-test/r/xml.result:
  - Fixing XML systax error in old test
  - Adding test cases.
mysql-test/t/xml.test:
  - Fixing XML systax error in old test
  - Adding test cases.
sql/item_xmlfunc.cc:
  Produce warning in case of XML systax error,
  instead of silentrly returning NULL.
strings/xml.c:
  - Making error messages better looking and clearer:
  It is important because now they're seen in SHOW WARNINGS
  (previously they were used only for debugging purposes).
  - Adding "unexpected END-OF-INPUT" error if after scanning
    closing tag for the root element some input is left
    (previously this error was ignored in a mistake).