The function receives an exactly-sized buffer (not a C NUL-terminated string)
and passes it into a printf function to be interpreted with "%s".

Instead, create an intermediate String object, and copy the data into it, 
and pass in a pointer to the String's NUL-terminated buffer.


mysql-test/r/warnings.result:
  Test that warnings do not read outside its intended memory space.
mysql-test/t/warnings.test:
  Test that warnings do not read outside its intended memory space.
sql/field.cc:
  Create a new String object and use a pointer to its data instead of the 
  exactly-sized buffer to be interpreted as a C string deep within the 
  errmsg.txt list via printf.