non-latin1 server error message

The problem was a one byte buffer overflow in the conversion
of a error message between character sets. Ahead of explaining
the problem further, some background information. Before an
error message is sent to the user, the message is converted
to the character set specified in the character_set_results
variable. For various reasons, this conversion might cause
the message to increase in length -- for example, if certain
characters can't be represented in the result character set.

If the final message length is greater than the maximum allowed
length of a error message (MYSQL_ERRMSG_SIZE), the message
is truncated. The message is also always null-terminated
regardless of the character set. The problem arises from this
null-termination. If a message length reached the maximum,
the terminating null character would be placed one byte past
the end of the message buffer.

The solution is to reserve the end of the message buffer for
the null character.

mysql-test/t/ctype_errors.test:
  Add test case for Bug#12736295.
sql/sql_error.cc:
  The to_end pointer was actually pointing past the end of
  the buffer. Since the message is always null terminated,
  point to_end to the last position of the buffer.