Update comment regarding MySQL user max length

The username can be up to 32 chars long as of MySQL 5.7.8

Author: kamil-tekiela

ext/mysqlnd/mysqlnd_enum_n_def.h

@@ -40,7 +40,7 @@
 #define MYSQLND_SQLSTATE_LENGTH		5
 #define MYSQLND_SQLSTATE_NULL		"00000"
 
-#define MYSQLND_MAX_ALLOWED_USER_LEN	252		/* 63 char * 4byte . MySQL supports now only 16 char, but let it be forward compatible */
+#define MYSQLND_MAX_ALLOWED_USER_LEN	252		/* 63 char * 4byte . MySQL supports now only 32 char, but let it be forward compatible */
 #define MYSQLND_MAX_ALLOWED_DB_LEN		1024	/* 256 char * 4byte. MySQL supports now only 64 char in the tables, but on the FS could be different. Forward compatible. */
 
 #define MYSQLND_NET_CMD_BUFFER_MIN_SIZE			4096