Problem: the scanner function tested for strings "<![CDATA[" and
"-->" without checking input string boundaries, which led to valgrind's
"Conditional jump or move depends on uninitialised value(s)" error.

Fix: Adding boundary checking.

  @ mysql-test/r/xml.result
  @ mysql-test/t/xml.test
  Adding test

  @ strings/xml.c
  Adding a helper function my_xml_parser_prefix_cmp(),
  with input string boundary check.