The slave IO thread sets MYSQL_SET_CHARSET_DIR. The code for this option
however is not thread-safe in sql-common/client.c. The value set is
temporarily written to mysys global variable `charsets-dir` and can be seen
by other threads running in parallel, which can result in use-after-free
error.

Problem was visible as random failures of test cases in suite multi_source
with Valgrind or MSAN.

Work-around by not setting this option for slave connect, it is redundant
anyway as it is just setting the default value.
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>