Problem: Memory overrun happened in attempts to generate
error messages (e.g. in case of incorrect XPath syntax).
Reason: set_if_bigger() was used instead of set_if_smaller().
Change: replacing wrong set_if_bigger() to set_if_smaller(),
and making minor additional code clean-ups.


mysql-test/r/xml.result:
  Adding test cases for all pieces of code with
  set_if_smaller() followed by my_printf_error().
mysql-test/t/xml.test:
  Adding test cases for all pieces of code with
  set_if_smaller() followed by my_printf_error().
sql/item_xmlfunc.cc:
  - fixing incorrect set_if_bigger to set_if_smaller in two places
  - getting read of unnesessary "char context[32]" variable and
    using '%.*s' instead if '%s' in the error format.