Problem: a byte behind the end of input string was read
in case of a broken XML not having a quote or doublequote
character closing a string value.

Fix: changing condition not to read behind the end of input string

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

  @ strings/xml.c
  When checking if the closing quote/doublequote was found,
  using p->cur[0] us unsafe, as p->cur can point to the byte after the value.
  Comparing p->cur to p->beg instead.