Fix incorrectly initialized character array

[Keyur]

Google's Address Sanitizer detected the following array out of bounds bug (http://code.google.com/p/address-sanitizer/wiki/ExampleGlobalOutOfBounds)

The log_warnings_suppress_name is read to initialize the Sys_var_set and the constructor depends on the NULL to detect the last element.

diff --git a/sql/sys_vars.cc b/sql/sys_vars.cc
index 4c98bed..76e3d7a 100644
--- a/sql/sys_vars.cc
+++ b/sql/sys_vars.cc
@@ -1597,7 +1597,7 @@ static Sys_var_uint Sys_port(
        READ_ONLY GLOBAL_VAR(mysqld_port), CMD_LINE(REQUIRED_ARG, 'P'),
        VALID_RANGE(0, UINT_MAX32), DEFAULT(0), BLOCK_SIZE(1));

-const char *log_warnings_suppress_name[]= { "1592" };
+const char *log_warnings_suppress_name[]= { "1592", NULL };
 static Sys_var_set Sys_log_warnings_suppress(
        "log_warnings_suppress",
        "disable logging of enumerated warnings: "